The solver can output volumes in two different ways but both use the OpenVDB standard. If you look at the settings under solver -> settings -> output you can find all the controls for the output.
The first mode is called VDB. This will output each field as a single VDB. Note that this mode is not multithreaded like the other mode.
Below that you can also output at a lower resolution than your sim division size by using the scale parameter if you want faster output when testing sim settings. Just remember to set it back when you are caching out your final simulation.
The second mode is called VDB MultiGrid. The difference between this and VDB is that this mode is multithreaded and the volume is broken up into multiple volumes for each field so you will see a field such as density broken up into many VDBs. The main reason you would choose this mode is because its much faster than VDB. The reason it needs to broken up into multiple volumes is because VDB creation is not thread safe when trying to write to a single volume. However, if you have each thread write to its own volume then its ok. The idea here is that you would output the multiple grids and then recombine them after simulation.
Note that you can recombine directly in the solver but I do not recommend it because the operation is almost as slow as just using the VDB output mode. I would recombine after you cache your sim.