Injector questions

[480sx]

I have a few questions about injectors and the terminology that goes with them.

I have heard that injectors dont care how you hook them up, positive or neg to either side. I have also heard that if your using used injectors, they polarize themselves, meaning that it does mater how you hook them up. Is this true? Also if it is, is there anyway to use an electrical tester to find out what side is/was positive?

On the top of injectors is a metal piece, almost looks like a button on mine. Do those have any other purpose other than to hold them in place?

Lastly, what exactly is an 'injector clip'. Searching on the net returns different results, some people have it listed as the plug itself and some as this little, thing piece of metal that somehow goes over the top of the injectors.