I've seen various opinions of what 'fill flash' is . My thoughts are that it is when the ambient is the mainsource and you only want to lift the shadows with a small amount of flash .
As you underexpose the ambient and use more flash you start tending toward 'fill ambient' where the flash is the main source and you 'drag the ' to expose some of the background as well .
So I just thought I'd share the graph I just made showing the relationship between , TTL flash and TTL-BL flash since defines TTL-BL as "Balanced fill flash" .
As we see TTL flash maintains the same output regardless of where you have the ambient exposed - it doesn't know or care how well you have the ambient exposed - it thinks it is the only source .
This is why we need to dial in more negative flash compensation when we use TTL flash and the subject is already properly exposed .
TTL-BL on the other hand 'watches the ' and in situations where they evenly [ a centralized subject filling the middle of the frame ] TTL-BL will back off to the equivalent of TTL -1.7 to simply act as 'fill flash' .
As we under-expose the ambient TTL-BL steps in and starts to take over control of the of the subject .
The graph I've shown is the result of some controlled tests I did using a flash to determine the difference in their output in varying situations .