In short, if you are simply informed about the connections between the fields, you are at level 2, but if you could discover the links yourself with no hints, you are at level 3. For example, if you know how the parameter “speed of light”, c, has implications for both general relativity and quantum phenomena, you have a level 2 understanding (to the extent that these fields are involved), but if you couldn’t discover the need for a “speed of light” parameter, how to find it, and how it affects the disparate fields, you haven’t reached level 3.
In short, if you are simply informed about the connections between the fields, you are at level 2, but if you could discover the links yourself with no hints, you are at level 3. For example, if you know how the parameter “speed of light”, c, has implications for both general relativity and quantum phenomena, you have a level 2 understanding (to the extent that these fields are involved), but if you couldn’t discover the need for a “speed of light” parameter, how to find it, and how it affects the disparate fields, you haven’t reached level 3.