Clarify what ##source is suppose to represent

[tcezard]

In VCF 4.3 specification in the first example the line ##source=myImputationProgramV3.1 is mentioned, but I cannot find this ##source field described anywhere in the document. Is it a non official field ? I think it should be either explained or removed from the example.

[husamia]

The source is not required at the moment. In the example is in silico source i.e. program. but for example if the caller is detecting somatic variants in cancerous tissue the source would indicate the details of the tissue source.

[lbergelson]

I think we could clarify in the example that ##source is an example of a program specific unstructured header line which is allowed but not defined in the spec.

[jmarshall]

Cf PR #620 which mentions ##source as an example of an unstructured meta-information line that is not defined by the spec.