Right now you have an APFS 'container' within which is an APFS 'volume', which is what you're booting from, using daily, etc.
The great thing about adding another volume to that same container is that they share the usable space of the overall container. You don't need to create a new partition and go through all the headaches associated with that. You simply create a new volume in the same container.
Example: you have a 1TB SSD, of which you're using 400GB (600GB free). You can create a new volume in that same container, and both volumes (your 'primary' one and your new/test one) have access to that 600GB of free space.
It's almost like they're co-existing. It's pretty slick, actually.
Here are some screenshots of my current setup.
#1: note that when I have the physical device selected, the blue bullet point shows both 'MojaveTest' and 'Machintosh HD'
View attachment 767987
#2: when I have the APFS container selected, you can see MojaveTest and Machintosh HD are in that same container. 269GB free for the whole container
View attachment 767984
#3: Macintosh HD selected. You can see it's using 699GB, 269GB free. This free space is usable by both volumes in this container.
View attachment 767983
#4 MojaveTest selected. You can see it's using 28.85GB, the same 269GB free.
View attachment 767982
Hope this helps!
[doublepost=1530121179][/doublepost]I don't have to decide how to split up my storage like when adding a partition. This is huge. All volumes use whatever's available. Adding a new volume is simple, removing a volume is simple, and neither operation requires resizing of partitions or anything. Any storage needs are handed out from the container dynamically.
I'm a big fan.