The project symbol path is a delimited list of common projects that Source Insight will search when looking up a symbol definition. It gets searched after all the open files, the current project, and the import library projects.
There is only one project symbol path that is searched regardless of what the current project is.
Every project in the project symbol path is searched. If the symbol is still not found, then the Base project is searched. If more than one symbol is found with the specified name, then Source Insight will ask you to pick from a list of the matching symbols, their locations, and types.
You can edit the project symbol path using the Options > Preferences: Symbol Lookups command. See: Symbol Lookup Options.
You can enable the Always search symbol path option in the Preferences: Symbol Lookups dialog box. When this option is on, all projects in the symbol path are searched every time Source Insight looks up a symbol, even if the symbol was already found in an open file or the current project. You may want to turn this option on if you want to see if there are redundant (or at least like-named) symbols in your project and any others on the project symbol path.
The project symbol path is only used to locate symbol definitions. It is not used to locate symbol references, or used by Search Files, or Smart Rename. Those operations only work on the files in the current project.