In Lacerte, you can often learn much by pressing F1 when you are at that input entry. The one you are questioning leads to::

Canada Article VXIII nonqualified compensation income

Enter the nonqualified compensation income related to Canada Article XVIII. This entry will be included on Form Schedule 1, other income, as a negative amount. Attach Form 8833 to the return to support this entry.

https://www.thetaxadviser.com/newsletters/2018/feb/tax-treaty-benefits-us-citizens-residents.html also gives some help - Look at Exceptions 5 & 7.

I am not familiar with this, but I think you found what you need.