Why another program uses .aup file ext?

Why another program uses Audacity project file extension for their plugins?

Audacity has no control over what other software developers choose to do.

A Google search came up with this:

The AUP file type is primarily associated with ‘Audacity’. Audacity is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems. Audacity projects are stored in an AUP file, which is a format that has been optimized for Audacity so that it can open and save projects quickly. Audacity breaks larger audio files into several smaller pieces and stores these pieces in a directory with a name similar to the project. The AUP format is not generally supported; projects must be exported to be played in other software.

Detailed information for file extension AUP:

Primary association: Audacity
File classification: Audio
Identifying characters Hex: 3C 3F 78 6D 6C 20 76 65 72 73 69 6F 6E 3D 22 31 2E 30 22 3F 3E 0A 3C 61 75 64 61 63 69 74 79 70 72 6F 6A 65 63 74 20 70 72 6F 6A 6E 61 6D 65 3D 22 , ASCII: ?xml version=“1.0”? audacityproject projname="

Other applications associated with file type AUP:

ABCgames Cheater (Update Library) by ABCgames.cz, AUP is update library of cheats, jokes or walkthroughs for ABCgames Cheater. , This association is classified as Database.
ePocrates File