Context Operator "In"...where can I learn the syntax?

Employee Year Month Count Vacation A 2015 August A 2016

August

B 2015 August B 2016 August

Thank you always in advance for your warm guidance and generous help...

My variable Count Vacation contains formula:

=Count([Vacation]) In ([Employee];[Year];[Month])

1.What does semicolon mean? Does order of dimensions matter?

2.How is it different from:

=Count([Vacation]) In ([Employee]) In ([Year]) In ([Month])

3. Is there any page where it explains IN context operators...?

Thank you...