SQL Segment Extensions question

There are two types of SQL editors to choose from when creating your SQL Segment Extension, and the editor you choose will determine the refresh. When you create extensions with the SQL Editor, you’ll manually refresh the extension, which uses up more SQL Segment credits. When you create extensions with the Incremental SQL Editor, it calculates only the last two days’ worth of data, which is more cost-efficient and uses up fewer SQL Segment credits. You can set incremental refresh SQL segments to automatically refresh membership daily.

What factors effect how many credits are used up per query and are there tips besides using the Incremental SQL Editor for minimizing credit usage per query? When you say the incremental editor only calculates the last two day's worth of data, does that mean if I have a segment using the people from this extension, only the last two days worth of users will be included, or will the new additional users be pulled into the segment every two days?