Historically, Ardour has always stored the names of the audio files in one of two ways. One would indicate that the file lived inside the session, and the other indicated that it lived outside, somewhere else in your computer’s filesystem. This meant that if you ever moved the location of the external files, Ardour wouldn’t be able to find them at startup, and if you wanted the session to work, you’d need to hand edit the session file.
Ardour 3.0 uses a search path for all the files (audio & MIDI) that it needs for the session, and if it can’t find them as it loads the session, it will ask you where to look for them. You can also edit the search path from the session properties dialog. There is no limit to the number of locations you can include in the search path.