The System attribute $File and File-type User attributes can link to a single file or folder per attribute. When displayed in the key attribute table, this attribute type has a folder icon to the left of the value box. Clicking the button allows setting a target folder or file. Or, if set, files are opened using Finder and folder are shown in Finder.
If the destination document may move around on the user's system, it may prove more useful to make an alias for the destination document, place the alias in a location where it will not 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.