The input 20-byte data is copied to the NS FIRM parameter buffer at offset 0x4B0.

This is used by Home Menu during TWL title-launch to pass a HMAC-SHA1 of the banner (size varies based on the banner version, etc.) to TWL_FIRM using the TWL and NTR SRL HMAC keys. This is presumably used for comparing against the HMAC stored in the whitelist for titles without RSA.