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 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.
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"
NFC management service "nfc:m"
This is used by the amiibo Settings applet.