tcman41's solution to your problem is the correct one, in my opinion. You won't see any performance improvement, given the size of your library, by splitting it into 2 libraries, and doing so will complicate your life.
You can use any free library field to implement tcman41's solution, including creating a new custom field. This latter approach may be better to avoid conflicting with other uses of the Keyword field. For example, a lot of podcasts populate Keyword.
With regard to your deeper questions, here is my attempt at an answer.
Physically, a library is a collection of *.jmd files located in a folder called "YourLibraryName".
Logically, a library is a database of metadata about files in your media collection to which each record in the database points.
A playlist is a collection of records in the database.
Put another way, an item in a playlist points to a record in the library which in turn points to your physical media file.
You can have as many libraries as you want, and each can contain any combination of your media files, however you may open only one library at a time. To share a playlist between 2 libraries you must export and import it, which is inconvenient, hence my suggestion that you probably want to avoid using 2 libraries.
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Topic: what is a library and what does it do? (Read 2679 times)