Hak paks are ERF files (with the extension .hak) that package custom content for use in specific modules. The name derives from the extension of the special file (.hak) as well as being a play on words (a variant on "hack pack", with "hack" implying modifications to the game, albeit with a negative connotation, and "pack" being short for "package").
Modules and servers utilizing hak paks cannot be played unless the player has installed the necessary hak paks on his or her own computer. The module's instructions or the server's web site must be referred to for information on which haks are required. Neverwinter Vault generally has most of the hak paks used. If a module does not require a certain hak pak, that hak pak will not be used while playing that module. (So having a large number of hak paks causes no problems, other than possibly disk space concerns.)
Installing a .hak file is simply a matter of placing it in the
hak subfolder of where Neverwinter Nights is installed. Some makers of hak paks simplify the installation even further by providing an installer.
Resources that are part of a hak pak override those of a module. This is perhaps most noticeable with scripts, but also can be relevant for object blueprints. That is, if one edits a script that is contained within a hak pak, (the Toolset gives a warning and) the edits will be saved within the module and subsequently ignored.
Hak paks are associated with a module using the "Custom Content" tab of the module's properties in the Toolset. The order of hak paks is important when multiple hak paks provide the same resource. (The version in the hak pak higher in the list takes precedence.) The maker of a hak pak will sometimes provide instructions on how to order the hak pak with respect to other hak paks.
The ability to associate a module with multiple hak paks was added in patch 1.28.