This part of IEC 61334 defines a set of encoding rules – the A-XDR3) encoding rules – that may be used to derive the specification of a transfer syntax for values of types defined in the DLMS core standard using the ASN.1 notation (see IEC 61334-4-41). These A-XDR encoding rules are also to be applied for decoding such a transfer syntax in order to identify the data values being transferred. The A-XDR encoding rules • are used at the time of communication; • provide optimal4) encoding for DLMS PDUs. NOTE Provided that A-XDR ensures optimal encoding for DLMS PDUs, it is intended to be the default encoding rule for DLMS-based communication protocols. Nevertheless, the default – and also the possibly usable optional – encoding rules will be specified in the Application Layer document of the given protocol (for example, IEC 61334-4-42), as part of the Application context