About Foreign Fields in VCF (4.3)

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?