In an action in my contract, I want to attach some text to it, so that users can see the text in their transaction history using a block explore like eosflare.io.

I see that eosio.token has a "memo" parameter that does exactly what I want, however when I read the source code, it doesn't seem to use the "memo" at all. How does a block explore know it is a memo? How can I also add a "memo" to my action?