The form for ITN has the following line
"Before you start: • Do not submit this form if you have, or are eligible to obtain, a social insurance number (SIN)"
As a non resident citizen you probably had SIN. Not sure how ITN was issued to you unless rules have changed recently.
One is to file a income tax return (ITN) as stated on the link I posted .
Example if you were undocumented and could not get a work permit to declare your earnings you could get a ITN to file taxes.
The CRA doesn’t care if you’re without status , their mandate is too collect taxes
Not all individuals filing a tax return can get a work permit
SIN is used for workers and filing tax returns
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