source - SourceKind

Description from IEC 61850-7-1:2011/A1:2020 Table 10

(default=process) Defines the source of a value. NOTE 1 Substitution may be done locally or via the communication services. In the second case, specific attributes with a FC=SV are used. NOTE 2 There are various means to clear a substitution. As an example, a substitution that was done following an invalid condition may be cleared automatically if the invalid condition is cleared. However, this is a local issue and therefore not within the scope of this standard.

source is a CODED ENUM (SourceKind) and can have two values

The server IED that is hosting the data can decide how to use this bit string. Its meaning is indicating that data is coming from a real process or is substituted somehow. One application field for this procedure is interlocking. To calculate the signal whether a switch gear is interlocked, input (position) from various switchgear is needed. Imagine one of the inputs is not sent for whatever reason. In this case you still need to set a default (you want to substitute) and then want to indicate that the outcome of the interlocking is based on a substituted value. The above source allows you to do so.

The additional advantage with this tag is that there is a service coming with it. You can not only define the default during the configuration of the IED, but you can also change this online with it is running.