Reg Project float.

Can someone help me understand if a project manager can build project float? I've encountered a practice question that talks about building project float of x weeks., and the key (answer) of this question seems to imply that this is a scheduling error.

Rita Mulcahy says Project float is the amount of time project can be delayed without delaying the 

(1) imposed date required by customer or management OR

(2) previously committed by project manager.