Can a tag hold data that is not NDEF?
Yes. NDEF is a convention rather than a physical constraint, and many chips allow direct access to their memory. What you lose is interoperability: content outside NDEF is meaningless to any reader that does not already know your layout, including the one built into the operating system.
Why it is possible at all
A tag is memory with an access protocol. NDEF is an agreement about how to arrange bytes within that memory so that strangers can interpret them, and nothing physically prevents you from arranging them differently.
Android exposes the underlying technologies individually, as its technology reference shows, so an application can read and write raw blocks on families that permit it rather than going through the NDEF layer at all.
What you give up
Everything that makes a tag useful to someone else. The operating system will not recognise the content, no generic tool will display it meaningfully, and no other developer's software will do anything with it.
You also give up the diagnostic ecosystem. When something goes wrong, a reading tool tells you the tag is unformatted or holds unknown data, which is true and unhelpful, and you are debugging alone.
For anything tapped by the public, this is disqualifying. A poster whose tag only works with your application is a poster that does nothing for the people who walk past it.
When it is legitimate
Closed systems where you control both ends. Your application, your tags, your readers, and no expectation that anything else will interact with them.
Compact binary data where NDEF overhead genuinely does not fit, which is a real constraint on the smallest chips and a rare one elsewhere.
Chip specific features that live outside the NDEF area entirely: configuration pages, counters, password areas. Using these is not really a choice between NDEF and raw access, since they are simply not part of the NDEF region.
Making both coexist
On many chips you can have both. NDEF occupies its defined region, and memory outside it holds whatever you like.
This is usually the right answer when you are tempted by raw access. A URL in NDEF gives everyone a sensible result, and your own application finds the extra data where it left it.
The tag then works for strangers and works better for you, which is a strictly better position than choosing one audience. It costs the NDEF overhead, and that is the price of being readable.
Frequently asked questions
Will a phone show anything for a non NDEF tag?
It will detect the tag and report its technology, then say there is nothing to act on. To a user that looks identical to a broken tag, which is the practical cost of leaving the convention.
Can I hide data on a tag this way?
Not from anyone who looks. Memory outside the NDEF region is readable by any application willing to read raw blocks, so this is obscurity rather than protection. Password protected areas are a different mechanism.
Do all tags allow raw memory access?
It depends on the chip family and on what the platform exposes. Some allow direct block access, others only through the NDEF layer, so the capability has to be confirmed for the specific chip.
Is a tag with both NDEF and raw data still standard?
Yes, as long as the NDEF region is correctly formed. Readers use the structure to find the NDEF content and ignore the rest, which is exactly what makes coexistence work.