A 'center flange' pulley has the mounting flange centered in the pulley.
An 'offset' pulley will have a mounting flange offset, like 3/4"
A standard pulley has the mounting flange at the end of the pulley.
Sometimes is beneficial to use an offset pulley instead of a spacer or a snout change.
For example, if your snout is too long, you can use an offset pulley to locate the upper pulley in line with the crank pulley. It's cheaper than replacing a snout.