If I can find a direct application of $maxlenl, which is to make sure that a string can be stored/used safely, it's not so clear what the others two are doing.

Item 28 of the latest beta suggests that they can be used to make sure that a given variable's name or window's name can be used safely but what features exactly correspond to each?

If you determine that one of the lengths applies to a certain feature in mIRC, such as channel names, then you will know to apply it in that situation if you want to. It would be impossible to document where they are used because they are used in thousands of places throughout the code. Of course, the string length types used by different features could also change in future, so there is no guarantee that they will always be the same. There is also no way to be sure that the string length corresponds to the actual maximum length of a server feature, eg. a channel name, since that depends on the server. All three lengths are provided simply in case you find them useful.

Ok, thanks.