Relative Item Selection notations

Dear All

I have a file on the relative item here by someone which works in creating a diagrid successfully across the building but I don’t understand the logic behind the mask used here. What does the following notation mean?
(I normally use {1;-1} etc only.

Many thanks in advance!
Kind regards
relative item notataion.3dm (252.7 KB)
relative item (10.0 KB)

It would be helpful to watch the linked video tutorial from data tree Masterclass by @DavidRutten.