Bleach Clash uses a Tier system to show the relative ability / Reiryoku levels between different characters. This isn't to say that you cannot defeat someone of a higher rank, it merely shows the advantage in strength one character has over another. A difference of one Rank shows a very slight advantage. A difference of one Tier shows a significant advantage. However, strategy will play a more vital part in combat than Rank (Within reason, of course).
To be read: Tier - Rank
Example: 2 - 3
Tier 1 - In Training
Someone at this level has only recently achieved their power. As such, it is often referred to as the "Training Tier". Users of this level go through the Ranking at a quick pace, the understanding of their abilities finally coming to the surface.
1-1: A character of this Rank actually gives off Reiatsu, and as such, has Reiryoku. Their abilities would have begun to surface. Example: Sado when his Fullbring first appeared.
1-2: A character of this Rank has gained a basic understanding of their abilities, and are capable of training them. Example: Shinigami seat 20-10.
1-3: A character of this Rank has trained their abilities, and is capable of basic combat while using that ability. Example: Shinigami Seat 9-4.
Tier 2 - The Lieutenant Level
This Tier is known as the Lieutenant level. Their abilities have been mastered to a high degree, and more powerful techniques are awaiting just around the corner.
2-1: A character of this Rank has begun to master their abilities. Due to this mastery, techniques have become more powerful and even more are to be gained. Example: Shinigami 3rd seat, new Lieutenant.
2-2: A character of this rank will have mastered their abilities and begun working on achieving a new level of power. These characters are capable of holding their own even against dangerous opponents. They may appear to be lucky, but they seem to be able to defy the odds and survive. Example: Omaeda, average Lieutenant.
2-3: A character of this Rank will have begun to gain a new understanding of their abilities and develop more in depth and powerful. Characters capable of a second release (Shinigami, Bount) will have begun the journey to unlocking them. Characters without will be truly strong at this rank. Example: Hisagi
Tier 3 - The Captain Level
This Tier is known as the Captain level. From newer to seasoned, these characters are amongst the strongest of their race. At this point, gaining strength is becoming harder than harder.
3-1: Characters of this rank will have either gained their second release (If applicable), or be capable of competing with it. They are unlikely to be able to hold their full strength for a long time, but when they use it they are a force to be reckoned with. Example: Renji
3-2: Characters of this rank are amongst the strongest of their race. Consisting of your average Gotei 13 Captain, to the Espada, this rank is the official "Captain Level". Second Releases (If applicable) can be held for a considerable length of time, and drain is near invisible. It is likely un-mastered. Other races will have developed complex and incredible abilities to counter even a Shinigami's Bankai. Example: Toshiro
3-3: A character of this rank are the Elite of the Elite. These are those who have Mastered their Second Release, as well as those who can fight against it. Example: Shunsui / Starrk
Tier 4 - The 'God' Tier
Characters of this Tier are considered gods amongst their Race. Likely to only be gained by one or two beings of a race in a generation, these are the elite of the elite. Gaining this Tier is tough, and progressing through this Tier is near impossible.
4-1: A character of this rank is considered frightening on all accounts. This sort of power is nearly impossible to fight against. These people can make even Captains struggle. Example: Yammy
4-2: A character of this Rank could fight against the Captain Commander at equal level, or even take on two Captain's at once. Example: Aizen, Yamamoto
4-3: Characters of this Rank appear once in multiple generations. It is unknown just how strong these beings are, only that the world shakes at their presence. Example: Aizen + Hogyoku.