Hi,

> Would that be
>
> ancestor-or-self::*[ObjectType][1]/@id

No, that you get you an ancestor node (or the current node) that has a
childnode with the element name ObjectType.

> What purpose does the [1] serve?

What ancester-or-self::*[@id] does, is to select the superset of all
ancestors and current node, which have an attribute called id.
The [1] here, then, selects the first of these nodes in reverse
document order - as that is the order of ancestor-or-self
(buttom-up)).
The last /@id selects the attribute id of that node.

What you would want was probably something like
ancestor-or-self::*[@id][self::ObjectType][1]/@id

I think you can write:
ancestor-or-self::ObjectType[@id][1]/@id
But this is not tested (I haven't had to do this before, and my
reference book is not nearby).

Hope this helps
Regards,
Ragulf Pickaxe :-)