Special Scenarios for spl screening A1, A2, B1, B2, c1, C2 and C3

Hi Experts,

Can any one explain about the special senarios for SPL screening like A1, A2, B1, B2, and C1, C2, and C3.

I have little bit confuse when we use these scenarios can any one explain in details if possible.

Thanks in advance for who explain briefly