FZ - 125kHz RFID
Learn & practice AWS Hacking:
HackTricks Training AWS Red Team Expert (ARTE)
Learn & practice GCP Hacking: 
HackTricks Training GCP Red Team Expert (GRTE)
Intro
For more info about how 125kHz tags work check:
Pentesting RFIDActions
For more info about these types of tags read this intro.
Read
Tries to read the card info. Then it can emulate them.
Note that some intercoms try to protect themselves from key duplication by sending a write command prior to reading. If the write succeeds, that tag is considered fake. When Flipper emulates RFID there is no way for the reader to distinguish it from the original one, so no such problems occur.
Add Manually
You can create fake cards in Flipper Zero indicating the data you manually and then emulate it.
IDs on cards
Some times, when you get a card you will find the ID (or part) of it written in the card visible.
EM Marin
For example in this EM-Marin card in the physical card is possible to read the last 3 of 5 bytes in clear. The other 2 can be brute-forced if you cannot read them from the card.
HID
Same happens in this HID card where only 2 out of 3 bytes can be found printed in the card
Emulate/Write
After copying a card or entering the ID manually it's possible to emulate it with Flipper Zero or write it in a real card.
References
Learn & practice AWS Hacking:HackTricks Training AWS Red Team Expert (ARTE)
Learn & practice GCP Hacking: HackTricks Training GCP Red Team Expert (GRTE)
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