About Foreign Fields in VCF (4.3)

Question

[Ibrahim Tanyalcin]

I was looking at the VCF format 4.3 here - page 7 and 4.2 page 4. More or less they are similar, stating 8 mandatory and fixed, tab delimited columns.

I was bit lost about the term 'fixed', I am wondering, say for another application, is it legal to add a foreign field, like "CustomXYZ" at the end, which would look like:

#CHROM                   CUSTOM
 ...           {{key1=value;key2=value..}}

This would look like key value pairs enclosed in handlebars ("{{..}}") which I do not see in VCF used at all.

Question is, would that be legit?

[WouterDeCoster]

Is there any reason why you would not add these key-value pairs to the INFO field?

[Ibrahim Tanyalcin]

Page 8-10 of VCF 4.3 Specification lists only two-letter fixed fields. I did not see in the spec, where they explicity say I can include arbitrary key=value pairs within INFO field. Is this assumption wrong?