OB81 Programming

hello guys

Im newly born baby in the world of PLC.I was studying the OB81 which is programmed for the battery indication.When i study the sample programme in the siemens manual i found two different variables one is OB81_FLT_ID and the other is OB81_EV_CLASS.I was really confused with two variables used at the same time.When OB81_FLT_ID can set the variable so why we use the OB81_EV_CLASS.So we can use only one varible OB81_FLT_ID to programme the OB81 or it is requrired the OB81_EV_CLASS.Plz if u have some idea plz share it with me.
Regard
Finskhan

OB81_EV_CLASS allows you to differentiate between "coming" and "going" faults.

OB81_FLT_ID tells you what the fault is.

Highlight OB81 in Simatic Manager and then press F1 to call up the Help file. There you'll find everything explained in the detail you need.