We can put a file in the A: drawer or C: drawer of our file cabinet. Similarly, we can store a file on the 95LX in the A: or C: drives. We store them in the "root directory", designated as A:\ or C:\ (note the backslash).
We can create in a file cabinet a thick file with a number of different, but related files in it. The fat file has a name and each of the related files have their own name. We can do the same thing in DOS by creating a "subdirectory" (sometimes simply referred to as a "directory"). A subdirectory is like the fat file among the normal files in our file cabinet.
Subdirectories are given names under the same conventions as a regular file. That means that a directory can have up to 12 characters including a period and a 3 character extension. In practice you will seldom find directories more than 8 characters long.
The 95LX comes with a built-in C drive subdirectory with the name _DAT. This is an arbitrary name. (HP seems to have named many of the HP 95LX's built-in files with the _ symbol as the first character. For example, if you look in FILER, at the bottom of the list of files, you'll see _STAT.WK1. This tells you that it is a built-in spread sheet that Hewlett Packard included as an unadvertised extra. We can assume that _DAT simply stands for data.
Whenever you save a MEMO, APPT, PHONE, or HP CALC equation file, by default you save it to the _DAT subdirectory. That means all your MEMO (TXT files), unless you otherwise designate, are automatically saved to your _DAT directory. Similarly, your PHONE and APPT files will be found in this subdirectory. If you had a RAM card, you could choose to store 1-2-3, MEMO, APPT, or your PHONE files on to the A drive.