The Practice Squad

"The difference between McCown and a practice-squad scrub is that only players with limited experience are eligible for the practice squad.  And a youngster like Moore isn’t a candidate for the practice squad because he likely wouldn’t make it through waivers (see Tyler Thigpen, whom the Vikings tried to sneak through to the practice squad in 2007 before the Chiefs intervened), and because once on the practice squad the young quarterback could be signed to any team’s active roster (see Brian St. Pierre, who was snatched by the Ravens from the Steelers a couple of years ago)." 

This was from ProFootballTalk.com  I would like for someone (Matty?) with great knowledge of the practice squad to tell me this - does someone on Miami's practice squad (once it's set this year) HAVE TO GO TO ANOTHER TEAM if the other team wants to sign him?  Can he not choose to stay with Miami and be part of Parcell's rebuilding process?  I'm familiar with terms but lack the knowledge of their application.   This practice squad thing - you can have up to 8 players but they might not make it through waivers - do you have to go?