From the Model Mayhem FAQ:

What are Verified Credits?
A Verified Credit is a two-way connection where both members verify they worked together on a project (i.e. a “collaboration”). By verifying a professional collaboration, each member builds a stronger network of trust within the Model Mayhem community and the modeling industry in general. Members can add details such as the number of times they have worked together, the date they last worked together, and praise for the other member where they describe in their own words what it was like to work together. This can also be edited later and re-verified if more recent work is completed.