Contract extension wizard and the Renegotiation wizard

What is the difference between the two. Do you have to get access to the contract extension wizard only through the special program it provides? I am a little confused with this.

Contract extension wizard is essentially team friendly deals and gives you a big unused salary cap over time.

Renegotiation wizard are player friendly deals and will run you over the cap.

Since there is no actual negotiation of contracts, the contract extension is your best bet. Makes FA more adventurous as well when everyone has money to spend.