Ronan,
CTO is the condition. Not part of the properties of a stamp. Part of the condition you give the stamp in your lists, or on the marketplace if you just want to sell it.
In the catalog you will find the stamp.
In the lists, you can add the condition after selecting it in your list (Collection, Wish or Swap).
On the sellers listing, you have to inditcate in what condition you offer the stamp (MNH, MH, Used or CTO).
There are collectors with an allergie for CTO (it is a product made just to sell to collectors, not to use as a stamp).
Others don't mind if the stamp is MH or CTO. As long as they can fill a gap.
The value of a CTO is always less than a stamp in the condition MNH or Used (really used).