Libraries
You can define multiple libraries in JRMC.
I have 1 home directory and 4 subdirectories for each major genre (blues, classical, pop, jazz)
I have one default library pointing to the home directory, displaying all the music.
When tagging I often switch to a specific library e.g. Classical as I use different conventions when tagging classical music.
It's reduced size compared with the default one also gives a better overview.
Creating a library
All libraries can be found in Playing Now
Add a Library
You can choose between an instance of the JRiver Library running somewhere in the network or create on on a HD.
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You can create a new one in your profile
Select the new library in Playing Now
In Tools > Import you can set the folders to monitor
Add as many folders as you want.
Rules
Having used different libraries for some time I become unhappy with this solution.
It takes time to switch from one to another.
If you have a top library covering them all and one per genre, each change in any of the sub libraries has to be synchronized and visa versa. This will be done automatically but it also takes some time.
If you define custom fields in a sub library you have to do it all over again in the top library.
Etc.
So I decided to maintain 1 and only 1 library.
However it is very convenient to have an overview by genre.
I did so by creating separate views for the genres by using the folder structure and a rule in the view.
This limits the view to everything in the specific folder.
If you create a new view e.g. Composer and drag it to Classical the same rule applies if you have the option "Use parent scheme rules for file display" checked
Custom Field
An alternative for using the folder structure is making a custom field e.g. Collection.
Tag your audio using this field and use a Rule to filter.
Thanks to wig