What does 'Tagged' mean in stamp collecting?

Answer 1

Tagged refers to a method of marking a stamp so that automatic mail machines can work with a letter. Early trials included graphite lines on the back of stamps. Today, most stamps are tagged with a phosphorus ink that is transparent and printed over the stamp design. Ultraviolet lights can detect them and allow the envelopes to be oriented to enable automatic cancelling machines and sorting machines to do their work.

Answer 2

Tagging refers to the use of various phosphorus inks to allow the machines to orient envelopes for cancellation. It also set them up for the optical address readers. The use of a blacklight is necessary to see tagging. In some cases, the tagging can be seen without the light. Some put it in the ink, others print bars or blocks on top of the stamp design.

Answer 3

Tagging is a phosphorus ink often applied to the stamps to allow automated machines to handle the mail. Mottled means that it is not an entire covering, but that the ink has blotched.