Most of the IPTC tags are text fields and can contain any text you want. To create a well sorted photo collection it helps a lot to care about the consistency of the tags before you start. I tagged 1000 photos and then decided to always use the singular (tree) instead of the plural form (trees).
With the masstagging feature you can scan your photos (in a directory and its subdirectories), see which tags are used in these photos (and how often) and edit some of the tags. For example replace all tags "trees" in all photos by "tree".
