New to apple trees I have a leaf question

tooln

5 year old buck +
When you guys talk about 2nd or 3rd leaf trees. What does this mean?
 
Number of years in the ground, planting year is 1st leaf.
 
OK that makes sense. Thanks
 
Can get confusing, when one guy has a three year old nursery tree and another has a 1 yr. But both planted in final spot in same year and both 1st leaf
 
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