I have found some answers like:
1. PSE stands for Partner, Students & Employees. this exam (VCP510PSE) can be taken by Partner, Students & Employees.
2. PSE stands for 'Professional Services Engineering' which is part of 'VMware Global Technology Solutions'.
But in none case I could fine a valid answer from VMWare. Also I would like to know if taking 2V0-621 PSE may be needed to take vSphere Foundations to get the VCP-DC?