A database / data entry term.
An Input mask controls what you can enter in a form field.
For example the input mask for a field might be set up like this ###-###
The hashes (may) mean that only numbers 0 to 9 can be entered and there will be a dash between the first three numbers and the last three numbers.
Exactly how you format the input mask depends on the system or application being used.
Input masks are very useful to make sure that data is being entered properly.
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