This wizard helps you to make quick choices of what ports to test with the IP addresses you selected.
Selecting 'Standard Ports' allows you to choose one or more commonly used ports. All selected ports will be tested with all selected IPs.
Selecting 'List Ports' presents you with a text field where you can list ports to test against all selected IPs.
If you choose 'Custom Ports', you will be presented with a text field for each IP you selected so the ports to test can be customized uniquely for each selected IP.