If you published a protocol in JMIR Res Protocols, your results paper is considered a RR2 (Registered Report Stage 2, see What is a Registered Report?).
To apply for a 20% discount on the Article Processing Fee (and also to make sure that the RR2 links back to the protocol (aka RR1), please enter the IRRID or manuscript number of the protocol in the RR2 field in the metadata form.
Related: