This "L0_YMD" directory contains an alternative directory tree structure where the date directories are converted into separate year, month, and day directories. For example, "20110417" becomes "2011/04/17".
It's important to note that this alternative directory tree structure is NOT compatible with the SECCHI SolarSoft routines. Those routines expect the directory tree structure in the "L0" directory, where year, month, and day are combined into a single string (e.g. "20110417").