The CPU menu choice lets you tell the program how often the program will run and how much memory the program can use.  See Setting Up Available Memory for more details.

 

The dialog box also displays your CPU type, speed, and capabilities.  This information is used to choose the best factoring and Lucas-Lehmer primality code to execute.  There are ways of overriding the calculated capabilities (see undoc.txt) if the program has misidentified your CPU.