# Difference in BJT Incremental Resistance between DC circuit and AC circuit

I'm trying to write up a lab report, but no one seems to be able to give me an answer for what the incremental resistance of a BJT means, or why it's significant so I'm hoping someone here can tell me. In our lab, we first built a DC circuit, pictured below:

We measured the incremental resistance by calculating Delta(Vy)/Delta(Iy) at different Iy values (10 uA, 100 uA, and 1000 uA).

Then, we built a second circuit, this time with an AC component:

We measured the incremental resistance again, this time with a more complicated formula, and the incremental resistance changed - a good amount, but it was still on the same magnitude as measured before.

I hope I'm not being ignorant by asking, but I truly do not understand the purpose of this laboratory and no one seems to be able to tell me - even the TAs. I'm assuming there must be some theoretical difference in the incremental resistance between AC and DC circuits, but I don't know what the theoretical explanation is. Additionally, I don't know what the significance of the incremental resistance is and/or what the "theoretical" incremental resistance should be for these two circuits. I'm not sure if I provided enough detail, but I'm really just looking for a good explanation of incremental resistance.