When you delete a character tag in Frame, it is only deleted from the catalog; text that has had the tag assigned to it is still "tagged".
A situation where you will come across this is when importing Word documents because Frame ends up with a character tag called Default Paragraph Font that doesn't really do anything -- when you delete it from the catalog, the status bar shows *Default Paragraph Font" (the asterisk indicates either that the tag assigned to the text is missing, or that the text is modified and doesn't match the catalog).
Another situation is where you decide to stop using a particular character tag, and you want to turn all instances of that tag back to the paragraph's default style.