In-store requisitions are available to anyone with 'Supervisor' access on the registers, under the GSTL 'K' key as you mentioned. You select the button for requisitions under K4 GSTL, then scan the items to be req'd as with any regular transaction. The difference is that with a requisition, the items are FREE charged to an expense account. 549 for TM recognition (prizes, best team survey schmoozing, etc.), 601 & 619 for supplies, 772 & 774 for charitable donations, and 979 for certain vibe situations. If you are a GSA/GSTL, you will most often use the 549 account to recognize your cashiers for REDcards. Other TLs/ETLs most often use 601/619 for their various supplies.
If you are a brand, pharmacy, HR or electronics TM who has been granted Supervisor access, do not requisition anything without LOD approval. These positions can be made Supervisor on the register for various reasons but are not actual 'supervisors,' and most likely have no need to ever perform a requisition (HR is sometimes excepted if the ETL-HR permits them to req food for the break room).