It’s the parts of a program’s concepts, rules and behaviours that are specific to the program’s task. For instance

• Items, a shopping cart and the conversion of such a cart into an order at checkout in an e-commerce application.
• Clips of video and audio, static images etc. and the compiling of these into a single output video for a video editor.
• Vertices, triangles, meshes, animation rigs etc. for a 3d editing program.
• Accounting standards and tax laws for an accounting system.

When developing software you deal both with these kinds of specifics and generically reusable concepts that are more purely computational science, so a term to distinguish them is handy.

Bugsnax also does this a bit with creatures like the Fryder.

The use of “alumni” in the singular. A person is an alumnus or an alumna, the alumni are always a group. Seems to be a very American usage, and I don’t know why it feels aggravating where other Americanisms like positive anymore don’t.