Funcion & Procedure - ???

i can use function or procedure in C2?

There's a couple of Plugins for using Functions in the Plugin Section

scirra.com/forum/plugins-for-construct-2_forum47.html

Be warned that custom plugins are not accepted by the Arcade

why the arcade not accept function?

security concerns

It's nothing against the Function object itself, it's just we won't add any old plugin to the Arcade unless we've done a security review, and then it is complicated to keep them up to date with the latest versions and so on. We don't have time for that right now, but we hope to be adding an automated third party plugin system soon to fix that.

so for now, i should not download and use behaviors that aren't on the updates C2?

If you only intend to upload your games to Scirra's arcade then yes, for now, keep to built-in plugins/behaviors only.