People use it to pair with NFC enabled devices which are basically just Android phones and tablets. These are using NFC Forum NDEF Tag Type 4 specification which is as you say NXP Mifare DESFire emulation in ISO14443 layer 4 APDU mode. If you have some reader (ISO14443 Type A PCD device) which isn't working with NFC "tag" emulation on nRF52 then please provide some details from 13MHz radio interface to be able to debug. If you cannot do that (which i do understand as ISO14443 analyzers/sniffers are bloody expensive unless you can hack some low-cost RF device for that purpose) then you will most probably need to switch to other reader/driver/middleware which allows you to debug on lower layer on that side.
Thanks for answer. My card reader finds a tag and works with it through APDU, but libfreefare a more convenient library can not find a tag and it's a pity. The documentation says that NFC Forum NDEF Tag Type 4 it's Mifare Desfire, but Android NFC Tool Pro application detect is as Mifare Plus and ATQA, SAK code corresponds to this. What card is actually emulated and is there a possibility to set an access key in this card?
Oh this has easy answer: read the (fucking) manual;) nRF52 chips which support "NFC tag" interface are simply able to act as ISO14443 Type A PICC up to layer 4. Nothing more and nothing less. So whatever application (and NFC Forum NDEF Tag Type 4 or MIFARE DESFire are some of these) can work on top of this it could be implemented. Because the nRF52 HW supports only basic routines to do anticollision and transport frames across the radio you need some "stack" or "library" on top of it. Nordic helps you with supplying two closed source (= just binary "blob") libraries which seems to work well in order to emulate NFC Forum NDEF Tags type 2 and 4 (NXP Mifare Ultralight and NXP Mifare DESFire). But you are free to implement your own if you don't like them.
