DCAU was basically created by Bruce Timm, the later stuff all built on what he started. Even if he wasn't the main artist he was basically the overlord of everything animated for DC, so as other artists came in they all emulated his style to make all the different shows/movies feel like they were a part of the same universe.
DCAU does not incorporate all DC Animated Projects just certain ones. mostly around Warner Bros Afternoon animation block and JL and JLU which sent to Cartoon network, still kind of bitter that Batman beyond ended early because of JL despite being the second most critically acclaimed & Awarded series in the DCAU. tho I keep purposely leaving out Gotham Girls from the DCAU cause A UGH... , B. it really doesn't add anything to discussion except some throwaway characters, C. Worst Animation out of the lot of them you can tell this one was cobbled together on the smallest Budget which isn't a surprise since it's blasted web cartoon.
so the following are the DCAU
Batman: Tas, it's sequel that everyone automatically lumps in with it
The New Batman Adventures, Batman: Mask of the Phantasm (loads of refrences to this in the original show), Batman & Mr. Freeze: Sub Zero (the Theatrical Finale for TAS before TNBA was made), Batman: Mystery of the Batwoman(takes place before the Flashback events of Return of the Joker, but after the three part episode Starcrossed in JL but before the episode Initiation in JLU) Batman & Harley Quinn (Bruce Timm says it is part of the DCAU the studio has failed to comment on if this is actually the case, tho there are references that are not in TAS)
Superman: Tas, Superman: Last Son of Krypton, The Batman Superman Movie: World's Finest, Superman: Brainiac Attacks.
Batman Beyond, Batman Beyond Return of the Joker (which the flash back scenes were a re-imagining of the Killing Joke comic, which would later have it's own animated movie in a similar but darker style, but was not canonical.)
Justice League, Justice League Unlimited
Static Shock (way to many cross over episodes. but adds another tech company besides star labs)
Project Zeta (really only ties into Batman Beyond)
Gotham Girls..... (because some twisted mind at the Studio thought this was a good idea to link this?)
Lobo (cause why not?)
and about 30 or so comic book series with way too many focusing on Poison Ivy and Harley Quinn
What's Not in the DCAU,
Teen Titans, Young Justice, any animated Movie that isn't Listed above.
Edit: For the Record, just because a Static Shock episode makes reference to the TT it was a reference to a cancelled project that got replaced with the TT we know, which I'm not sure how it would of went as the JL Pilot has Cyborg as a Girl.