For now, you'll have to refer to the example filters as found in the current report specification files. We'll give one other example here: specifying a time range.
Suppose you want to be able to report on only a specific time range. You could build a (possibly global and reused) filter like:
<filter-spec> <and> <ge arg1="$timestamp" arg2="$period-start"/> <le arg1="$period-end" arg2="$timestamp"/> </and> </filter-spec>