Explaining how to operate or install a Umsdos system is not enough. Most people are seeking some advises about using Umsdos or not.
The goal of Umsdos was to ease the installation of Linux. An other goal was to ease its UN-installation. The idea here was to promote the spreading of Linux. Installing a new OS on a system is always troublesome. OS/2 for one will happily pollute your
C: root with a bunch of new directories. If you are clever like me, it will also erase your config.sys and autoexec.bat files :-(
The pseudo-root feature of Umsdos avoid this unwanted invasion. Linux can be UN-install without side effect.
If you have a small hard drive, Umsdos will allow you to share disk space between DOS and Linux. A disk below 300 megs is in my opinion a small disk. This opinion is based on the size of the different package available today. One popular word processor may eat as much as 70 megabytes if you select all features.
If you have a larger drive, you may consider having a dedicated Linux partition running the Ext2 file-system. Ext2 use a smaller cluster size that DOS (1k in fact) so installing many small files will eat less space than in a Umsdos partition.
The following point apply to Umsdos compared with Ext2.
FATlocated at the beginning of the hard drive. The DOS file-system is probably more fragile because of this.