Tree tags are a useful item for physically identifying individual trees with a unique number. However they are only really essential on sites with high numbers of trees located close together. Sites with few trees can usually be identified easily from just a site plan.
The tag number can be used as a reference within a survey schedule and used to identify the tree on the topographical survey plan.
A commonly used tag is the aluminium disc which is either oval or circular in design with a diameter of approximately 30mm (I prefer the oval shape because it enables the easy alignment of holes for threading together when you inevitably drop a few). The number is embossed and holes are punched for the fixing nails. I personally use a staple gun to fix a staple over each hole, its quicker and easier (try holding a small nail in one hand, hammer in the other, on a freezing cold day with clip board clenched between teeth). The tags are supplied, strung together in loops with thin wire. They are numbered 0001-9999 and available in sets of 100 or 1000.
Another type of tag is the Latschbacher 'no-nail' system which uses a special hammer to apply the numbered coloured plastic square tag to the tree. Tags are stored in a magazine styled rack. Once applied to the tree special lugs secure the tag in place. Tags can be ordered with numbers from 01-10000.