Using a text attribute as a condition - contains vs is
So I was making a hangman type template and in my solution actors, I put a rule that when an active letter IS the letter of that solution then it revealed the letter (display text). The active letter being a text attribute changed by clicking a particular letter. For some reason, considering each attribute is literally a single letter and both in capital letters, it did not work. However when I changed it to contain, it worked. Which leads me to my question(s):
What is the difference between IS and Contains? How are they are functioning? And does capitalization of letters matter?
I ask because IS by my understanding would be the equivalent of "exact" and CONTAINS would be the presence of within a grouping or attribute etc. IS being more accurate and contains for when I want a wider selection. I don't understand why there is discrepancy, if anything both should be working.