Difference between Anchor Base and Scrap Relative

Hi Team,

Having a scanned PDF, read a value using Anchor base activity and it worked totally fine, although it doesnt give me output if we scroll on same page little above or down.

Read the same scanned PDF using Scrap relative ,and it working perfectly fine even when i changes resolution and scrolling done.

Could you tell be when we need to use anchor base and scrap relative. And its difference.