When you create a style using Style Manager, the style is saved only in the current drawing. If your style-based content needs to be available for use in multiple drawings on a regular basis, create the style in a new or existing drawing that you can use as a library for permanent storage.
For example, if you want to have a library of symbols to use throughout a series of drawings for project ABC, open a new drawing called Project ABC Ortho Iso.dwg and create all of the styles in that drawing. The Project ABC Ortho Iso.dwg file acts as your style-based content library. Naming the drawing according to whether it contains symbols with orthographic and isometric views, or symbols with only orthographic views, helps users during symbol selection.
As you draw a building system, you can access the style-based content in a library if the library drawing is located in a folder specified as a default location for style-based content, or in one of its subfolders.
The following folder contains a set of customizable style-based content libraries: \ProgramData\Autodesk\MEP <version>\enu\MEPContent. The libraries have unique drawing names based on the type of content they contain.