When and Why does G0 phase occur?

I’ve asked this question, specifically because i’ve seen this question.

As we see in this diagram , G0 phase occurs after M phase and at a specific point within G1 phase.

Is there a meaning to saying "end" in this case?

What is the checkpoint for causing a cell to enter G0 instead of continuing with the cell cycle, in case of heart cells and neurons? (what exactly is "checked"?).