The left side is the position that definitions of intelligence are all arbitrary, and that psychologists just make up tests and call what it measures "intelligence."

The middle is the position that there is a real thing that can be called "intelligence," which can be defined in different (meaningful) ways, and that intelligence tests are objective ways to measure it.

The right side is the position that intelligence is probably still real and can probably still be defined in different (meaningful) ways, but that we can never directly measure intelligence and instead observe it indirectly through observable indicators like someone's performance on an intelligence test. This means that any practical statement about intelligence, while probably real and definable, are contingent on the specific test used to measure it.