Hell Neo,
This field is mainly meant for defining the Importance of an Equipment master.
A - Very Important or Critical (You can say Breakdown of this could hamper total process)
B - Important (The impact on process is moderate)
C - Common Equipment (Impact on process is negligible)
Accordingly, after classification like this, the reports can also be retrieved according to this, by giving these values in the selection screens of tcodes like IW28, IE05, IH08 etc. You can get have important PMIS reports on the basis of this field (MCI1 to MCI9) for analysis of statistics. You can have graphs here (in PMIS) also in ALV reports (IW28 etc) on the basis of this field values.
Regards
KJogeswaraRao