How it is calculated
Each series in the Encyclopedia has a list of pieces. The ones in your collection count as obtained. The rest are shown as pieces still to get, with their entry.
What you see in each series
Your progress in pieces, the list of the ones you have and the ones you are missing, and a direct link to the active listings for each. Pieces that have not come out yet appear as upcoming.
Choosing the series you follow
You do not need to follow them all. Mark the series you collect and the rest will not appear here.
A tip on method
Start with short series. Completing a series of a few pieces gives you a collection that makes sense and teaches you to read entries before buying.
Purchases and progress
Pieces you buy on GSG are added to your progress automatically. The ones you own from other sources you mark yourself from the entry, with the Add to my collection button.