What are the differences between sequential, random and dynamic access modes in COBOL?

Answer 1

Sequential access is pretty much what the name suggests: start with the first record in the file and read each successive one in order.

Random access files have one or more indexes so that, given the record's key, that specific record can be accessed directly without having to step thru the records preceding it.

Dynamic access is kind of a combination of the two...you can use index key data to position to a particular spot in the file and switch to sequential reads as desired, mixing random and sequential reads as much as you need.