I think the default is to apply equal weight to each term in the box (not really an "AND", but not really an "OR", either).
This document explains it in more detail:
Thank for your Answer.
I don't find in this documentation anything for searching two or more terms without Operator respectively without quotation marks.
The result of a search inculde results with only the first term and results of only the second term. That's in my Opinion an OR query.
EDIT: @ Matthew
Sorry, you are right. As a first all results are displayed with two terms.
More and more people are building their careers with Atlassian, and we want you to be at the front of this wave! Important Dates Start the Certification Prep Course by 2 April 2019 Take your e...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs