SOLID Principles
Dear class, you have one job. Do it or we debug you.
"open for extension, closed for modifications"
Can add more classes but abide by the current client implementation.
The parent class can be replaced by the child class without changing the desired property of the program.
Clients should not know of methods they don't need. TLDR, mind your own contract (interface). Let other clients do the other contracts.
Similar to ISP but in a sense that you can only see the contract but not behind the scenes (the implementation).