Two types of existence?

Hello Rabbi.
On page 74 of the first book.
You distinguish between two types of girls in your mind:
1) Definition and understanding in the mind (without any connection to reality)
2) A view (or some perception of reality) and from it a definition for understanding in the mind.
And you say that if you defined something and then saw it in reality, it is not the same definition of the concept (such as the triangle).
Why separate? If I defined something, and then saw it in reality:
1) Or is it the same thing I defined (so why say the definition is different)
2) This is not what I defined, then I create a new definition and say – "Exists"
My question is why separate one definition that simply exists/does not exist (before/after) into two different definitions?