When using Rules to set a value on a taxonomy term reference field, I get choices like "field:0", "field:1", etc. What do these numbers mean? When I add a "Set a data value" action for my taxonomy term reference field, Rules allows me to choose from the following:


field-age
field-age:0
field-age:1
field-age:2
field-age:3


What are these ":0" ":1" etc. fields?

EDIT: These appeared when I changed the "Number of values" for field-age from "1" to "2".  However, since I only changed the number of values to 2, why did 4 things appear?
It's basically letting you have access to a zero-based array for the different instances of a multiple cardinality field (as I think you've already worked out).

It seems the reason there are 4 shown is just a default; a visual cue to let you know there are more than one possible values for that field. 

If you have a field with 20 values you'll still only see 0...3 in the Rules data selector, even though you can use 0...19.