What is the "Subject" Field for?

The short answer is: the "Subject" field is for whatever purpose you want to use it for.

That being said, some people use the subject field to describe the task that they were doing; however, that's not how XPunch was designed. There is an entire structure of Clients > Projects > Tasks in place to facilitate sorting of tasks in reports.

The subject field was designed to provide users with the ability to enter additional general, overall information about the task being completed.