MLS designated player rule

My proposal for the research project is the MLS designated player rule.  I do not know much about this rule but I know that it has brought some of the MLS's greatest players into the league.  I would like to find out:

-The salary cap for these designated players

-What a team has to go through to get a designated player

-Why this rule exist  (why are teams limited to these types of players)

-Identify these designated players and see if they really stand out

-The difference between a designated player and average, normal player

This topic interest me because the "designated player rule" has brought in many great players into the MLS.  Without this rule, the league is empty.  Legendary players like Pirlo, Kaka, and David Villa would have never graced these fields.  Although these players are out of their prime, their past achievements show how good they really are and bring more popularity to MLS soccer.