@jbarwick
golfball posted a link earlier of a you tube lecture regarding these assertions. What I extracted from the lecture that made sense to me was:
occurrence assertion = looking for documented proof
completeness assertion = confirming that all #s are accounted for.
1. You are starting from the source documents (shipping orders) and tracing the flow of that information through the accounting system to ensure that it has made it to the accounting records (sales journal) confirming it has been recorded as sales (the accounting records are complete)
2. You are starting from the accounting record and looking for the documented proof that that sale really occurred
Hope this helps a little.