Minor is basically upgrading to minor service release i.e. 7.5.1 to 7.5.2, etc
Major would be going from 8.0 to 8.5 for example
Migration is a vague term as there are many types of migrations, it could be hardware migration to new or existing version, migration to new data center, migration of side B to different data center, manual migration of the solution vs. upgrade due to the fact that the software release is so old and would require too many hops, etc
The UCCE upgrade guide have some more details, if you are interested further.
HTH, please rate all helpful posts!
Chris
