Relationship to DevonThinkPro Office & other products?

As a (very) new user, I am in the process of getting to grips with DevonThinkPro Office: Is there an explanation somewhere of the relationship between DEVONsphere Express and the other DevonThink products?

In particular is DEVONsphere Express complimentary to DevonThinkPro Office, or a subset of DTPO?

Many thanks.

DEVONthink, DEVONagent and DEVONsphere are based on the same technology but don’t require each other and are therefore complimentary.