[@Yankee] So, this is actually a really good question, thanks for asking that.

For two 'normal' ranks, like A and B, I always see them as rough gauges rather than absolute statements. Not every Servant with B Strength will be 100% equal in an arm-wrestle, for instance. And at the same time, if you're really low on energy, it might be possible for you to lose an arm-wrestle to someone that your parameters say you're stronger than. 

Also, we see in canon that the parameters are inclusive of a few different things. For instance, Agility includes dexterity, but also movement speed (Kojiro from Fate/Stay Night has higher Agility than Cu Chulainn, but Cu Chulainn is faster). So it's possible for a Servant to have sky-high Agility, but that refers largely to their dexterity and their movement speed is actually pretty low, like with Kojiro.

This is a bit of a messy system, but the way I think I'm going to handle it is by treating the parameters as those rough guidelines, but as generally pretty 'fixed'. If a Servant with higher strength gets into a direct physical clash with another, the former will win. With Agility specifically, I'll also be using the classes as a way of deciding movement speed (we know Lancers are generally faster, for example, so in a foot race, a Lancer could beat another Servant with equal or even higher Agility).

If a Servant is injured or otherwise not in top performance, that'll be a little different and I'll decide things like that more case-by-case, since it's hard to make hard rules for something so situational.

For + and -, we actually know what they mean in canon!

So, a + on parameters means that usually your parameter's the base rank, but in set circumstances, it can go higher for a moment. For example, Mordred's B+ strength means her strength is usually B (which has a value of 40), but when she bursts really hard, she can double that to 80 for a moment. With skills and Noble Phantasms, it's kind of similar, but instead of being instantaneous, the +s are more like conditions. For instance, Semiramis has C+ Presence Concealment, which means that her usual Presence Concealment is just C, but the + lets her stay concealed even when she's poisoning stuff. Or if you had a Servant with B+ Item Construction who specializes in making swords, then you could say that your usual ability is B, but when you're making swords, the + is active and it's actually above even A rank.

A - is a little easier to explain. It means that in terms of 'rank' you're at that level, but it's kind of unstable, and in practice it's usually a little below that rank or maybe even one full rank lower. For instance, Mordred has C- Charisma, and can only show her full potential in a rebellion. That means if she's leading a rebellion, she has C Charisma, but if she's not, then it's actually a little below C or closer to D rank.

This is all a little long-winded, but I hope that helps clear things up!

[@King Cosmos] Got it!