The System attribute $File and File-type User attributes can link to a single file or folder per attribute. Only $File has the button in the note sidebar to open the linked asset). The open action actually opens the linked assets via Finder so a Pages Doc will open in Pages, and HTML file in Safari, a folder in Finder, etc.
If the target document may move around on the user's system, it may prove more useful to make an alias for the target document, place the alias in a location where it won't get moved and link the alias to the Tinderbox attribute. If the alias method is used for many notes/files, it can prove useful to create a single Finder folder for all these aliases and thus stop the aliases themselves from being moved and so breaking the Tinderbox attribute's link.
As stated, the link can be to a folder. Using the latter allows a single note/attribute to link to a collection of files, for instance a folder containing all files relating to a particular project.