NFC Services
The New3DS NFC module was added with 8.1.0-0_New3DS. The Old3DS NFC module was added with 9.3.0-X.
On New3DS NFC module uses the NFC hardware via the i2c::NFC and gpio:NFC services. On Old3DS NFC module communicates with a NFC peripheral via IR with the IRUSER service.
A total of 6 sessions can be open simultaneously for all of these services combined.
NFC user service "nfc:u"
This is the NFC service used by regular applications. This was first seen in the Super Smash Bros eShop demo(only in the exheader, the demo doesn't actually use it), at that time no system-module was available for NFC on CDN.
At the time of writing, there is no titles using this service yet.
NFC service "nfc:r"
NFC service "nfc:p"
This service is used by the mint library-applet, starting with 9.3.0-21. This service was added to the mint service-access-control list with 9.0.0-20.
NFC service "nfc:s"
NFC development service "nfc:dev"
This service seems to be intended for use only on dev-units(or at least some of the command(s)).
NFC management service "nfc:m"
This is used by the amiibo Settings applet.