Support offers - confused by Onsite service differences

I am going to buy several Optiplex 7000 tower PCs.

I am confused by the differences in Onsite Services.

I see the following offers:

1. Basic Onsite Service
2. Hardware Service with Onsite Service After Remote Diagnosis
3. ProSupport with Next Business Day Onsite Service
4. ProSupport Plus with Next Business Day Onsite Service

What are the differences in these 4 offers??