Adding a line to a ucs, uca or ai file that you want ZT to ignore.

[fern]

The preferred way of doing this is to use a semi-colon (;)
I use this to put the name of my building in the ai file at the top under the id. It helps me when I have files side by side comparing what I have done or want to do.

[D66640F1]
;Small Bear House

Blue Fang examples:

These are common ones in ingame scenery files
;Bad Habitat Type
cHabitat = 9414

[EstheticBonus]
; man
v = 9503
v = 15
; woman
v = 9504
v = 15
; boy
v = 9505
v = 15
; girl
v = 9506
v = 15

in buildings:
[slots]
name=slot

[slot]
; using
;id=9503
; would restrict this slot only to "man"

; spot in the building where guest would play animation if didn't disappear
; (in world coordinates, relative to building center; 16 per half-subtile)
slotpos=-16
slotpos=48

; spots outside the building where guest paths to
entrpos=-16
entrpos=48

entrpos=16
entrpos=48

; spot to place guest in once they've used the building
exitpos=0
exitpos=48

; 1000 = essentially infinite capacity
capacity=2
facing=0

As noted by Rick about another one used:

//comment

I got my model from animal ucas where it says

[m/Characteristics/Integers]
//Reserved for new prey animals
cAllCrowdedChange = -5
etc...

According to Jay, // should not cause any problems because ZT ignores anything it does not understand. Also according to Jay, there are occasional // in uca files, but even in those ; is used most of the time. So that is what is preferred.