The difference between a relation and a function is that a relationship can have many outputs for a single input, but a function has a single input for a single output. This is the basic factor to differentiate between relation and function. Relations are used, so those model concepts are formed.
A relation can have many outputs for a single input but a function has a single output for a single input. For example if there’s (2,4), and (3,4) that’s a relation but a function can’t have any repeating numbers on either point
