ANDKeeps messages that match all of the criteria.
AND between two criteria requires both to be true. Google's example pairs a sender and a recipient: from:amy AND to:david.
from:amy AND to:davidOpen in Gmail ↗Messages from Amy addressed to David.
AND is required or whether a plain space is enough: its other examples (in:archive payment reminder) chain criteria without AND.Not documented by Google · Gmail Help: search operators