Modular vs Block IO Modules?

This may seem like a basic question, but could anyone shine light on the main difference between Block and Modular IO.

More specifically, the difference between AB FlexIO and AB's Block IO options.

I have experience using local chassis IO, and remote FlexIO modules over a variety of networks; however I've never encountered or worked with Block IO (say 179-CompactBlock), and the AB website and documentation is a little too verbose for me to make a clear distinction between the two.

Thanks!

Block I/O is a single piece of hardware that has the I/O Communications adapter and inputs/outputs in one device. It is designed as a ready made solution to hook up and go. Some types of block I/O can be mounted out in the open, with either conduit entries or IP67 rated connectors for all connections.

Modular I/O (remote racks, Flex, Point) has individual parts, the communications adapter and then whatever I/O modules you might need. Modular I/O must almost always be in an enclosure, as it isn't generally as ruggadized as block I/O's can be.

Thanks, that sums it up nicely. I'll read up some more on the documentation...