[Under the Covers]

Welcome to MR!

Right-click on the Tags column, choose Add your own columns. From there, you can use the + button (on the right) or the Add Custom Column bar (across the bottom) to add your column. The resulting dialog box allows you to name and set the type of column. Return from the dialog box to the first dialog. Then save that, close Calibre, and restart. I found that I needed to drag my column title to the location I wanted following the restart, even after re-ordering the list in the first dialog box.

As to what you want to call it and what type of column to choose, others may come along to make suggestions, or you can just play with it yourself to see what works for you.

ALSO: There is a separate Calibre forum here, which has all kinds of helpful tips and guidance. I would encourage you to read through applicable threads.