Pattern Grammar is a model for describing the syntactic environments of individual lexical items, derived from studying their occurrences in authentic linguistic corpora.
It was developed by Hunston, Francis, and Manning as part of the COBUILD project.
[1] Each word has a set of patterns assigned to it which describe typical contexts in which they are used.
The word skim includes the following patterns in the COBUILD dictionary: Skim is a verb, and as the pattern describes its behaviour it is in upper case.
The choice of preposition is limited to those two options, which is why they are specified directly instead of using the broader category 'prep'.