Upload Meter Readings Difference between FM's

HI Gurus,

I want to know about the difference between FUCNTION modules BAPI MTRREADDOC UPLOAD and ISU_S_METERREAD_CHANGE...

currently for us the functionality is that if a periodic meter reading/meter reading order exists on the same day as the incoming interim meter reading(09), then the FM ISU_S_METERREAD_CHANGE is used to upload the meter readings or else if no periodic meter reading /meter reading order is found then the fm BAPI MTRREADDOC UPLOAD is used for storing the meter readings. I want to know why these separate FM's are used, is there any difference in the way they store the meter readings?

Need a bit of urgent help..:)

Thanks a lot!,

Suman