It is a convention that the variable or type name may be dropped if the delimiters themselves can carry that meaning, that is, if the delimiters surround only one kind of tag. For example,
<au John Palsgrave>might mean the same thing. In the first form, single diamond brackets are the delimiters that separate the variable-type "au" and the value-token "John Palsgrave" from the text. In the second form, the double diamond brackets are understood to stand for "<au >" rather than "title" or "date." Any other tags must use different and unique delimiters.
All COCOA tags apply to the text from whatever they occur and hold until another tag with the same variable appears. That is, every word in the text following "<au John Palsgrave>" would be tagged as being written by Palsgrave until a subsequent "<au >" tag occurred. COCOA-style tags have several implementations and no fixed set of rules. <i>TACT</i>, for example, can employ them but by relaxing the length of variable names in tags in effect makes its own version of COCOA tagging available.