Under USPS standards they are typically the secondary address type designation (e.g APT, BLDG, STE, etc) and the number or code for that type; for example when taken together APT C, BLDG 2, STE 101, etc.
There's a whole set of standards for how USPS address should be formatted. Most folks don't know what they are and good programming will parse what's entered on address lines 1 and or 2 and format them appropriately for machine readability rather than prompting for each component separately.
