OP_RETURN code has some interesting, but highly controversial, applications. It’s used to mark a transaction output as invalid, allowing users to burn bitcoin, its also used to add some data to a transaction or store data on the blockchain, which you shouldn’t do.

Some use OP_RETURN prefixes as a way of confirming that they are behind this transaction, using colored coins or storing data.

Recently there were discussions on how to claim these OP prefixes but some members of the community thought it was a bad idea as it would allow censorship and reduce anonymity.

This relates to an old BIP from 2016, but work on this BIP is still in progress.