It's the man who beat the man. Which of course traces back for many years.
If the Lineage is vacant, it can be re-determined by the universal #1 Vs. the universal #2.
The "Lineage" is often open to interpretation, as far as I'm aware the "Ring Title" is their interpretation of "The man" and the "Lineal Champion".
But, other's people may differ to there's I.e Mosley's WW Lineage in 09-10. Many considered him the Lineal Champion whereas The Ring and other's felt he wasn't.
So, it does get confusing at times.
That's how I look at it anyway and always have.