No difference, that is just what remington calls thier chokes I believe. It probably says what size it is on it as well. I don't think rem choke is a size, but rather the brand name. If you only got one choke with the gun my guess would be it is a modified choke. I think that is the standard with the 870's. Mossberg 500's come with all three chokes.

You can buy any after market choke that will fit a remington and use it as well.

Paul