A blank is flexible, but the letter it represents is worth zero.
Assign the letter when you place it
Select the blank marked ? from your rack and place it on the board. Choose a letter in the picker. That choice stays with the tile after you submit the move. Recall lets you change an unsubmitted placement.
CAT with a blank A
Through the opening centre square, this version scores 8 rather than 10. A double-letter or triple-letter square still multiplies the blank's zero by two or three.
A word premium still works
A newly placed blank on a double-word square doubles the complete word. Later turns may use that blank as its assigned letter, but its letter value remains zero and the used word premium does not apply again.
Keep it or spend it?
Check what the blank makes possible before playing it in a short word. It can connect your rack to an awkward letter on the board or let you use all seven tiles. There is no fixed rule to keep it until the end.
Use ? in Word Finder to explore the same rack with a blank.