Here's some more info on what constitutes a "series". Generally, a series should meet the following criteria:

Publisher Recognized. This means that the series should in someway be "official". A trilogy or items (or a 4-item or 5-book series) is the prime example. Likewise, a coherent, publisher recognized series of modules (e.g., B1-12).

Recognizable Megaseries. Even when you're creating a megaseries, it should in some way be recognizable. A series of trilogies set in a coherent timeline might be one reason for a megaseries, but remember the next rule.

No Replication. A series should not replicate another datum in the index (e.g., "Game System", "Setting"). So, no "Transhuman Space" or "GURPS" or "Glorantha" series. Basically, if you can already get the precise listing via a simple search, it doesn't need to be here.

Single Series (Usually). Usually a item should only be in one series. There are two notable exceptions. First, when the publisher has really marketed a item as part of multiple series.(For example, "Crown of Fire" was sold both as "The Harpers item 9" and "Shandril's Saga Book 2".) Second, when a book is a sub-series that isn't part of a coherent set of sub-series. (For example, B11 and B12 form a 2-part adventure, but are also part of the B1-12 series, and the rest of the B items don't neatly cohere into subseries as was the case for the TAGDQ items. Thus both the B1-12 and B11-12 series should be input.)