An Order To Show Cause was filed by Kevin Federline's attorney on Wednesday which is seeking some change to the custody or visitation agreement. Reports have indicated that Kevin has filed for "Primary Custody" but that is not actually anything you can file for. People often confuse custody with visitation but they are separate issues. It is very rare for one party to be awarded Sole Legal and Physical Custody and it really has nothing to do with visitation. Generally the only time a party who has sole physical custody can keep another parent from visiting their child is if there are domestic violence issues or if the life of the child is in danger. To give an example, Charlie Sheen and Denise Richards have joint legal and physical custody but Charlie only has supervised visitation. In his new request to the court he wants to modify the visitation, not the custody.
I haven't seen the documents Kevin's attorney filed, but would imagine that Kevin will be requesting a modification to the visitation order which has been in place since the divorce was filed. An Order to Show Cause (OSC) is basically a document served on another party that says give me a reason why the court shouldn't give me exactly what I'm asking for in this document.
The document usually contains what the moving party (Kevin) is asking for. California law also requires that mediation occur before any permanent changes can be made. Often an Order To Show Cause is accompanied with Temporary Orders that the moving party (Kevin) wants put in place until a hearing can be held. The Temporary Orders could ask for something like Britney can't have any visitation. In most counties, Temporary Orders are rarely granted unless the life of the child is in danger.