When do you absolutely have to declare a class as abstract (as opposed to free-willed educated choice or decision based on UML diagram)?

Imagine you have some set of inter related classes like mango , apple ,banana all these classes are having same functionality so instead of creating again repeated methods in these classes i will create a more generalized method fruits and in that i define the idea means how to do it and later on sub classes will implement those ideas so in a company many people will work for 2 or 3 years and leave the company we cant take the code of them what they have written i mean to say in projects many people mainly project managers create abstract classes and later on sub ordinates will use or override the function of that idea which is present in abstract class like that it works.when project are very big we just modify the code in abstract class it will automatically modified in sub class it is like kind of relationship eg : mango is kind of fruit like that and once i declare a class abstract i have to override it if i declare it virtual then it is upto me to override or not .Now coming to interface when classes are not related like cycle , train , bus means bus is belonging to automobile class and cycle to some other class but these different classes want to implement common functionality then interface is used and it is used to overcome the draw back of multiple inheritance as i can implement multiple interfaces and there is another drawback in inheritance using base class object i cannot call the sub class function to overcome that abstract classes are used .Try this stuff using code you will understand better thank you read less