What's a tag?
An OFPEC tag is a three to five letter identifier that is unique to each designer and is used as a prefix to variables in addons and scripts to avoid global namespace conflicts with other addons and scripts.
All tags will be followed by an underscore (whether you include one or not) and that's how they should be used when naming variables and files: TAG_namespace.
PBO Signature Keys
In Arma, there is a method of verifying addon integrity through signed PBOs, which uses a key system. With the Private key you can sign your addons, and the Public keys can be downloaded by server owners to ensure addons are 100% genuine.
Everything you need to know about OFPEC Tags...