Hi Thierno,
As Amlan and William pointed out, SAP has defined standard Meter reading reasons which you can see in EL31. These reasons are used by SAP programs in different processes. So reasons are used for Meter reading entry which you can see in EL01, Some used in Business process as William mentioned of Disconnection, Device Modification etc, and some are used for Interpolation of meter readings.(check SPRO for DM->Meter Reading->Meter Reading Order->Order Creation->Define Priority of MR Reasons for Interval Check).
You can define your own reasons too but keep in mind that the standard SAP program would not use them,so generally not recommended. Check Tables TE609,TE610,TE611.
Hope this helps. 
Cheers,
Rakesh..
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