What is reader writer mode?
Reader writer mode is the phone acting as the powered side: it generates the field, wakes a passive tag and exchanges data with it. It covers reading, writing, formatting and locking. It is also the only mode where an ordinary application has genuine freedom to do what it wants.
What the phone does
Everything, energetically speaking. It produces the field, supplies the tag with the power it needs to run, sends every command, and interprets every answer. The tag contributes nothing but a response.
Within that, the operations available are reading content, writing new content, formatting a blank tag, setting a password where the chip supports one, and permanently locking the memory.
The only mode you control
Card emulation is gated by the platform and, for payment, by the card schemes. Peer to peer no longer exists in practice. Reader writer mode is what is left, and it is genuinely open.
An ordinary application can read any tag it can talk to, write standard content, and inspect technical details. Nobody has to approve what you put on your own tag. For anyone building something with NFC rather than merely using it, this mode is the entire playing field.
Where the platforms differ
Android exposes the technologies directly, and its reference documentation lists them individually, from the low level radio layers up to NDEF operations. An application can go as deep as the hardware allows.
iOS is more constrained. Core NFC provides tag reading and writing within Apple's rules, and the application generally has to be running and in the foreground for a session to start. In practical terms, on iOS the user has to open the application first, which changes how an interaction can be designed.
The difference matters at design time rather than at read time. A workflow that assumes the tag will be tapped with no application open will behave very differently on the two platforms.
What it still cannot do
Read a tag that uses a proprietary protocol the controller does not implement. Reach data protected by keys you do not hold. Change a factory programmed identifier on a standard tag. Rewrite a tag that has been locked.
None of these are software limitations you can work around, and the last one is worth repeating because it catches people out permanently: locking is a one way operation, and reader writer mode is the tool that performs it.
Frequently asked questions
Do I need a special application to use this mode?
For simple reading, both platforms handle common content without any application at all. Writing, formatting and locking need a tool, since neither operating system exposes those operations in its own interface.
Can two applications read the same tag at once?
No. One exchange, one application. The operating system decides which one receives the tag, based on the content and on which applications have registered an interest in it.
Why does iOS need the app open when Android does not?
It is a platform decision about when an application may start an NFC session. Background reading on iPhone handles some content without an application, but an application that wants to control the exchange has to be running.
Is writing a tag riskier than reading one?
Yes, in one specific sense: a write interrupted partway through can leave a tag in an inconsistent state, and locking cannot be undone. Reading a tag cannot damage it.