Advanced Consisting - Also known as Decoder Assisted Consisting, this uses a temporary secondary address in the locomotive to group locomotives together without changing the primary address.
The consist will use a single slot in the command station. It is limited to a two digit address.
Advanced Consisting is done by using ops mode programming to change configuration variable 19 to set the address and normal direction of movement. This also requires reprogramming the decoder to change the consist, but some systems do this transparently to the user. Some decoders will allow you to specify what functions will respond to commands addressed at the consist address.
If you have a small number of slots in your system, say 10, then the command station sees the consist as a single locomotive instead of several. This saves slots (or memory) in the Command Station.
Also if you'd take your loco SET to another layout and they have a different system than you, then their system will can just call up the Advanced Consist number (CV19) and run your set and you don't need to re-consist your locos.
If their system is only set up with 2 digit addressing, their system can talk to your CV19 address as it's a two digit address.
