Question about potential regarding Primary and Secondary Lines.

I've always been a bit confused about this. When you have potential on an item, I always hear people refer to certain lines as primary or secondary. What exactly does this mean? Is the primary line just the line with the best stats? Thanks.

Primary is the line at the top, usually.
Ex. 9% Int 6% Int 6% Int on a Lvl 71+ item that is Unique. The 9% Int is the primary line.

It is possible to get multiple primary lines though and not that uncommon anymore either.
Ex. 9% Int 9% Int 6% Int on a Lvl 71+ item that is Unique. Both of the 9% Int lines are primary lines.

Anyway, some defines the first line as primary line and others as secondary.

The logic of it goes like this:

If your first line rank up, your equip potential rank up as well.

If any lines but the first line rank up, the potential rank remains.

Hence, the first line is the primary line.