ItemHTMLFile contains only a file name, not a pathname - IQ looks for the file in the .Files folder. .Files folder is set in Option 2.4.10, and is stored in <database name>.ini in the same folder as the corresponding .sndb database file.
ItemHTMLFile contains both a pathname and filename - IQ finds the file at the designated path and filename.
FileName has only a filename, no pathname, and FilePath is empty - IQ finds the file if it is located in the .Files folder. Image Viewer displayed an image file I put into the .Files folder and referenced this way.
FilePath or ItemHTMLFile contains network pathnames in form \\computername\sharename\pathname\filename - IQ can find the linked file from different computers on the network. Handy when using synced database is loaded into IQ on different network computers, and the database contains links to files residing on computers other than the one on which IQ is running. In fact wouldn't it be better to use a network path whenever possible, if the database is to be used by different computers on the network?
ItemHTMLFile refers to a file on another computer, using a local file path - Doc pane displays a file not found error message.
FilePath refers to a folder on another computer, using a local file path - Image Viewer displays a "File Not Found" message in its title bar.
FilePath refers to a folder on another computer, using a local file path - nothing happens the item is double-clicked to load the linked file. I didn't see an error message.
Comments
I use it because I store all my documents on OneDrive, and depending on the computer I'm using, the path can be on either the C: or D: drive.
Haven't tested it with FilePath but would expect it to behave in the same way.
Don't use unusual characters in your folder names. I had links in the doc pane breaking because I had an apostrophe in one of my path strings.