When using an external TTL dedicated flash unit, are your cameras flash settings such as Auto Flash, Slow Flash, Slow Flash 2, controlled by the external flash unit also, or are they still functioning independently.
It would be helpful if you told us what camera brand/model and flash you are referring to.
TTL = Through The Lens, which means the camera body metering is controlling the flash exposure, be it the one built into the camera or an "external" one mounted in the hot shoe.
If by "external" you mean a slave flash not physically connected to the camera communicating with the camera, that requires a "master / commander" flash to signal the slave via pre-flashes. Some Canon and Nikon bodies allow the built-in flash to serve as the master. Some functions many not be possible with slave flashes.
Camera is Olympus E series DSLR ...the flash is a Vivitar DF 383 mounted onto the cameras Hotshoe flash is set in TTL mode. what I would like to know is when operating this way can the cameras flash settings still be ajusted such as to slow flash, auto, Etc. Or is the TTL mode controlling everything!.
If you have the gear the simplest way to find out what works and what doesn't is to try each of the features you mention, first with the built-in flash, then with the Vivitar, then compare the results.
I am far from any authority on Flash but I think from what your question sounds like you are thinking the TTL Hotshoe flash is controlling your camera, I believe it's the other way around, the cameras TTL is controlling the Flash! so some settings on your camera that are ajustable may still be useable.
