Board Thread:Questions and Answers/@comment-30238140-20161015204322/@comment-24659969-20161015232154

Ethanlac wrote: The higher your User Rank is (the total level of all Cats and base upgrades), the higher level limits go. Eventually, with a high enough User Rank, you will be able to upgrade Basic Cats to level 20+60. The cap now is: Normal 20+60 Special 30+0 except some has 30+10 Rare 30+70 Super and Uber rare: 30+60

Any tier other than normal can be further upgraded with catseyes up to 40+XX mentioned above.

Since it seems you are still newbie, just complete the first 3 main chapters of Empire of Cats story then clear some of Stories of Legends and Into the Future. You will understand it eventually.