In the GNU GRUB info pages in the Ubuntu 16.04 LTS distro, the naming convention for partitions seemed straight forward with the following definition:
Here, 'hd' means it is a hard disk drive. The first integer '0' indicates the drive number, that is, the first hard disk, the string 'msdos' indicates the partition scheme, while the second integer, '2', indicates the partition number...
...This expression means the second partition of the first hard disk drive. In this case, GRUB uses one partition of the disk, instead of the whole disk.
This would seem to indicate that the first hard drive has a fat partition of some sort on the second disk partition and is referred to as an msdos partition scheme.
Reading on, there is another example with the following text:
This means the BSD 'a' partition on first PC slice number of the second hard disk.
Since the first definition specifies that the second field is the partition scheme followed by the partition index (starting from 1), and section 13.1 makes reference to the ability to specify "sub partitions", then this example seems to indicate that the bsd1 partition is a sub partition of the msdos1 partition.
The text indicates it is the "BSD 'a' partition" on the "first PC slice number". I am not familiar with the "BSD 'a' partition", the term "first PC slice number", or the concept of sub partitions so this makes it a bit confusing as I am unable to find further information.
What is a sub partition and how does it differ from logical and extended partitions? More specifically: what is a BSD 'a' partition and a first PC slice number?