Pay codes are the "categories" that wages, reimbursements, and deductions are placed into. They are a core payroll concept that consist of:

1. A human-readable name. Shown on reports, pay stubs, and elsewhere.
2. A string "key", used as a constant identifier for API integrations and other workflows.
3. A type indicator, which controls the behavior of amounts using this pay code.

Each company has its own list of pay codes, and also comes configured with a built-in list of pay codes. Generally, you should use a separate pay code for each wage, reimbursement, and deduction type, to ensure the numbers for each type are tracked and displayed separately.

Pay codes are referenced in these APIs (the Payables API, for instance) using their apiKey value, which is an alphanumeric string that LOOKS_LIKE_THIS.