You can create sections directly in Content Library. When content is created in the content library, it becomes the default, or master, version.
When that section is added to a proposal or a template, another version is created, and the default/master will not be affected. You'll see a second version appear in your Revision History. Any edits you make to that version will only affect the version in that proposal or template.
If you edit the version in the Content Library, you'll be editing the default version of that section.
Creating a Section in your Proposal/Template:
When you create a new section in a proposal or template, this first version is your default/master version. When editing that version in your proposal or template, you will be editing the default.
If you edit this section in the Content Library, this creates a new version, (separate from the one in your proposal or template) and becomes the new default.
Or, if the new section you created in your proposal or template is added to another proposal, a second version is created. Editing the second version in the new proposal does not effect the default.
At anytime you can set the default to another version, by clicking "Make Default" on a version in your Revision History. This will become the one used when importing and it will be edited in the content library.
See also Revision History/Versions