Hello, after the games with Patch I noticed that the tool quite advanced… However it is very difficult to me to understand the principles of this Patch 
In the beginning I have a question - why the surface from Patch has problems with Join (sometimes) in what a problem? And if to use different curves in Patch, then almost not to foresee result on what to be under construction the tool? What do I have to know about Patch? I have no task, I need to understand Patch more clearly…
Hello - Patch makes a surface that tries to match up to any arbitrarily shaped border - if the border is complex and the number of spans too small in the patch (i.e. not enough control in the surface to be able to meet all the input requirements) , then the edge of the patch cannot come close enough to the input to be able to join. With more spans, it may be able to come closer to the edges. I agree Rhino could help more…
https://mcneel.myjetbrains.com/youtrack/issue/RH-17058
https://mcneel.myjetbrains.com/youtrack/issue/RH-4181
-Pascal
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Hi Pascal, a surface from Patch has no accuracy specified in the project?
By the way, Patch at a large number of fields (more than 100) program demand many RAM - it is normal? If to create a surface on a grid of curves - there is an opportunity to create surfaces with a huge number of fields and problems don’t.
Also it would be desirable that @Toshiaki_Takano, @Dale, @ec2638, @Brian, @RichardZ have looked at a subject
One way to control match would be creating points or curves that guides the way patch is made.
Tho it might make things worse it terms of matching the surface…
If patch can show which surfaces it can match to, and perhaps a different combination of which edges, it may be useful. The edges that can’t be closed needs manual work but at least you can select which one you want to tackle. Sort of priority of patched edge…
I tried to use different curves for the help of Patch, but they periodically spoil smoothness of the created surface (for example, weak dent). What can you tell about it?
And what to do if at enough passes of a surface nevertheless don’t want to unite? 
Curves good - I have checked also edges not difficult.
patch is generally a little tricky to get under control, i have omitted this function before since the surfaces are difficult to join, but have started playing with it recently again trying to unriddle its success.
there are certain rules i have found to be helpful:
if the patch is simple then a low amount of spans and a low stiffness (far below 1) will suffice. best is to start with the lowest possible stiffness and slightly increase the spans till it fits.
for complex shapes this will not work because it will warp the shape very strong, here generally more spans and increasing the stiffness far beyond 1 will help getting an understanding of a shape, but it will not necessarily close up anymore.
Huge thanks to you!
I have lowered stiffness to 0.008, now there is enough small quantity of spans, but everything unites also without any problems 
The problem is solved, but all the same thanks!
I don’t know if this is a good solution…
When all else fails… trimming the patch and also the surface you want to patch then just blend surface between.
Looks quite messy in the attached file, but at least no naked edges with minimal effort and some control over the surface shape…
Unable to join.3dm (1.0 MB)
I tried to use curves for the help of a surface from Patch - it is useful for good control over geometry 
However then to have to make it is a lot of different actions, clicks - it doesn’t suit me, and here through Patch I can create very difficult surface and editing stiffness, spans for good association.
Patch always goes haywire when I select starting surface. It is very difficult to control border of the surface made by the Patch command. Sometimes I go through several iterations - I make temporary curve by duplicating the border (DupBorder), I edit control points of the temporary curve, and I use the curve as input in the next iteration.
I personally don’t understand at all how to use function of a starting surface… In practice?
Can you attach the file?
Apparently I made mistake by using annular surface. Big surface without holes seems to work fine.
I doubt that the file will help you understand how to use the surface. When you start the Patch command, you do not select the surface. You select it later, when the Patch window appears - it is one of the options in the window.
If you attach the file, then I will be able to define usefulness of function.
Hello - here’s a simple example of where that might be useful.
The resulting patch takes the structure of the starting surface, the number of spans in the dialog is ignored,
Patch_SimpleStartingSurface.3dm (74.5 KB)
This can also be used by using a plane as a starting surface - if the starting surface is a planar degree 1 by 1 surface or a plane, the size and orientation of the starting surface is used with the number spans set in the dialog.
-Pascal
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I used “input curves” and “trim curve” as input of the Patch command to make “first patch surface”. (I did not use the “starting surface” as input.) The “second patch surface” and “third patch surface” used “input curves” and “temporary curve” as input of the Patch command. I made the temporary curve by mirroring the border of the “first patch surface”. I changed the “temporary curve” before making the “third patch surface”. As you can see in the attached file, the “second patch surface” is better than the “first patch surface” because its border is closer to the “trim curve”. The “third patch surface” is not much better than the “second patch surface”.
patch.3dm (294.1 KB)
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Ok. Thus it is possible even to remake the closed objects 
Thanks for the attached file! Good reception!
One trick that may be useful: If you want to make the Patch surface better approximate a curve, make multiple copies of the curve (all at the same place). When the Patch process asks you to select the curves to be approximated, select all of the copies. (If you do this for all the various curves, then the multiple copies trick probably does nothing.)
As for starting surface, and whether it is a 1x1 plane … gee whiz, McNeel could say something about this in the documentation. I suggest there be a second section in the description of each command, called “details”.