Hi,
In short,
Current System : Your 5.1 version
New System : Your 5.7 version
Base System : A version in between these two which contain functionality of both.
i.e. if upgrading from r99 to 5.9, then Current System is r99, New system is 5.7 and the Base system should be 3.1.
This is from my point of understanding, and also when Pivotal upgrades, then they use all the versions in between. i.e when upgrading from r99 to 5.9, their path will be r99 - 2.0 - 3.1 - 5.1 - 5.7 and so on.
So in upgrading from 5.1. to 5.7, i am still not very sure if these is a Pivotal version in between to become a base system.
