Here's a bit more information/clarity:
- Built-in display panel: Always controlled by the integrated GPU, and this cannot be changed. The discrete GPU can still accelerate graphics on this panel by operating as a render-only device, but it cannot directly control the panel.
- VGA output wired to underside dock connector: Same as above.
- DisplayPort and DVI outputs wired to underside dock connector: By default, both controlled by the discrete GPU, but the "Enable Dock Display Port" BIOS setting allows DVI/DP #1 to be directly controlled by the integrated GPU. This can be handy if you're using the built-in panel and a single external display together. DVI/DP #2 are always controlled by the discrete GPU.
- All built-in display outputs, including Thunderbolt 3 / USB-C: By default, controlled by integrated GPU, but the "Special Graphics Mode" allows the discrete GPU to control them instead.
NOTE: It seems that if you disable "Switchable Graphics" completely, you may be able to get the internal panel and VGA outputs to be driven by the discrete GPU, but forum posts indicate that at least on the 7510, this means that you cannot undock while the system is asleep and have the built-in display start working. Instead, you have to shut the system down before undocking.