In the element spec for <app>, the descriptions of both @from and @to could use some revising:
1. The descriptions end with ", if necessary", but this is redundant since they are already optional attributes. So it should be removed.
2. The allowed value is "any valid identifier", yet the datatype is "data.pointer".
To resolve (2), we either need to:
a) change the datatype
or else we should both
b) change the "any valid identifier" to "The identifier of a node" and
c) change "identifies" in each description to "gives the identifier of the node which is".