Relevance of Liable to Tax Phrase set out in OECD Model Treaty

What is the importance of the term ‘Liable to Tax’ as set out in the OECD Model Treaty?

The resident in question could posses a tax liability .

Charities and or a government can be liable to tax and posses a tax liability that is later reduced or eliminated by provisions relating to their status.

For example, a charitable organization in the US could lose its favorable tax treatment status. Thus having to pay its actual tax liability.

A person who does not pay any tax because of loss carryovers or deductions is also liable to tax. Their losses or deductions serve to reduce their liability.