Quote from: pooya87 on June 07, 2018, 04:40:55 AM

- you can include some data in your transactions using the OP_Return code which is limited to 40 bytes
https://en.bitcoin.it/wiki/OP_RETURN
https://bitcoin.stackexchange.com/questions/29554/explanation-of-what-an-op-return-transaction-looks-like

This rings a bell, the past illegal contents in the blockchain issue can be linked to that "OP_Return" function.
I don't know how it works but based on your statement, it is possible to insert "something" in to the blockchain.

Quote from: pxstein on June 07, 2018, 03:54:39 AM

In case it is for internal use only: Is there another way to pass some small additional info text ?

pooya87 was right, including a "description" wont increase the total Transaction size.

You can send a link to the receiver a blockexplorer which supports Meta Data addition like blockcypher.
The Data will be stored to their site's server, not the blockchain. The problem is, anyone can add their own MetaData to that tx as well.
Like this link for example (scroll to the bottom to view): Transaction with Meta Data.

P.S: There's a link below the page which said "You can also embed data into the Bitcoin blockchain" which I suspect that is "OP_Return", anyone can confirm this?