Bib uses the word fatigue to describe the appropriate target. I think its a good one. The penis isn’t a muscle but so it doesn’t get fatigued the same way a muscle does. It fatigues more like rubber and steel.
But the generalized mild soreness in the ligament and tunica areas are probably a sign that you are doing productive PE. Kind of like a muscle feels “burny” after its been worked hard. This is because a cascade of chemicals has been released generally to the area as a result of damaged cells. Some guys might call this pain. I’d consider this the yellow-light area that you should achieve occasionally. With experience you might even push into this area every time you PE. But it also means you’ve approached the red area.
Sharp pains that are localized, that you can actually feel a specific point from which the pain emanates means you’ve gone too far. You’ve been close to injury. You may have also created a weak point that can get much worse with even less stress than you’re used to.
So there are varying levels of pain. No burning sensation and you can’t be sure you’ve pushed into the effective stress range. Occasionally dull burning (similar to lactic acid burning in muscle) and you can be sure you’ve been causing cell damage, which is what we’re after. Sharp pains and point tenderness (hurts when you touch it) and you’ve probably gone too far and you need some recovery.
Given that we can vary, some guy’s yellow zones are pretty broad and others’ might be just shy of injury. You need to approach PE slowly enough that you locate your yellow zone well before you inadvertently go red. And you can’t be sure you have a broad yellow Zone or a narrow one.
That’s my description any way. The word “pain” is too general.
All that said. I found a link that had an equation on the failure points of ligaments. A ligament with a 5mm cross section has a failure weight under slow application of that weight of over 200 lbs. We hear of injuries here semi-frequently but never about guys tearing a lig or ripping open the tunica itself. That still doesn’t mean that relatively low stress can’t damage sheaths that surround ligaments and collagen, nerves and blood vessels enough to cause real trauma and lasting functional effects. In short PE is relatively safe from “ripping your dick off” or “making it explode” but you can hurt yourself and your ability to fuck for months.