The ZZT-OOP engine has a very limited capability to interact with external text files. These must be explicitly given a name via the Hyperlink command or they will default to HLP.
There is a per-file limit of 1024 lines. Lines can only be 43 characters long to still have a space at the beginning and end of each line.
Tabbing does not work; all spacing must be done manually. Extra characters will overflow the text box to the right-hand side and cause graphical abnormalities.
Besides presenting text via the in-game text windows they can also be used to present menu options to the user. For this to work the object and label names in the text file must both use capital letters.
An example of a valid .hlp file from Terrain.hlp bundled with ZZT version 2.0:
Neither players nor creatues can
travel through water, but bullets pass
freely over it. The player must
maneuver around water, taking care to
avoid bullets passing over it.
From ZZT versions 3.0 and onward, the contents of the .hlp files were integrated with the game program itself instead of being seperate external files.