When are Jira workflow conditions actually evaluated?

[Adolfo Casari]

I am trying to find out when are workflow conditions actually being evaluated. For instance:

a) When issue page is loaded

b) When user clicks on a transition

etc.

Thanks in advance.

[Nic Brough -Adaptavist-]

Have a look at b).  A user can't click a transition that is not there. 

Conditions hide transitions if a user can't do them.