Explaining the terms:
Rank F,
Afk stands for Away Form Keyboard.With no doubt this is the worst rank ever,which means this guy is almost useless to your team.And it occupies a space,while this space can let one player to join and fight.

Rank D,
Random Acting/No purpose,it means this guy may shoot enemy,or more often to walk around,enjoying the landscape....digging around to find some diamonds to make diamond sword,hence d for dig!

Rank C,
Just as explained

Rank B,
Realizing the goal refers to the game's goal.If it's flag mode,attacking or defensing the flag is the first priority than killing enemy.You don't have to understand perks and weapons in deep,but briefly.

Rank A,
Able to conjure doesn't like what you read,I just simplify it for understanding,as pressing the spacebar also called conjure.In fact it means you can control conjuring,you don't have to control it well.The basic thing is you can fly yourself with conjuring.As flying is too important in this game.(fly not=conjure jump)

Rank S,
Well,it's hard to define what do I mean by[unique skill].In idea,sometimes you see ppl doing some abnormal ridiculous thing to defeat ppl,which terrifies you,then it's what I call [unique skill].

---

List of skill
Dashing front with conj(conjure)
Focusing at front with conj which you can't stop him by shooting him
Glitching through walls or conj
Conj jump(CJ)
Switching guns when no ammo
Other than skills,they are:
Strategy
Intelligence
Reaction
Ping(Lag)
and #!@$ Hack
In fact strategy includes many things,including:
Choosing the right weapon for different game mode
Choosing a right place to stay and fight
Excelling at perks
Excelling at conj
Trapping enemy
Keeping your sight minimum enemy
Forecasting enemy
Considering your team
etc....

Or using the 5W1H model to help you think, what who where when why
What do you want to deal with enemies?what weapons,perks?What is the goal?
Who is fighting with you,against with you?Who are you?VIP?Then escape from any enemy.
Where are you going to stay and shoot?Where are the enemies?Is this a safe place or dangeous place?
When can you use next perk?When is the game finish?Reload now,or 10 seconds later?
Why are you doing this?Is that really important?What about the second plan?
You have a plan,but how to do it?
etc...

In PVP,
Rank C^ VS rank F=100% win
Any Rank VS the same Rank=about 50% win
Rank D VS Rank D+1(means C)=about 40% win,so
Rank X VS Rank X+1=about 40%win
Rank X VS Rank X+2=about 30%win
Rank S includes anyone who is above Rank A,hence it can't measure how far between one s rank from another rank S. e.g one s rank VS another s rank with many unique skills will usually lose.

---

In VIP,as said before
Being VIP B is required,A^ is recommended.If he's below B,then hope the team can help him,or the enemy VIP is also the same rank or below.(However F,an afk VIP may be better than D or C,true story)

And the average rank of your team,C is required,hence you can at least stay with them to protect you a bit.And B^ is recommended.
A rumour said,no matter how well the VIP is hiding,he will see an S rank from enemy at least once(If there has one).

---

In Flag,
Rank C is required,B^ is recommended.However some maps require you to fly up,so A^ is required in those maps.As C or below has nothing to do with the flag while they can't even see it.

---

In domination,
Don't want to talk about it,it's unhealthy at the moment.

---

The ratio of rank S/A/B/C/D,personally I guess
S15%/A15%/B20%/C30%/D20%
(In fact there has no rank F,just a joke.If someone is rank F he shouldn't even exist in the game)
Winning makes you confident in such competitive game,so higher rank of players occupy a bit larger in the ratio.