Thank you for the response David.

It means that an OSLC query cannot express a condition based on the existence or non-existence of a property, regardless of whether that property is a string, integer, enum, URI etc. If you have a requirement to query for the existence of some property (any value), then that use case is currently not supported by the OSLC Query specification.

See https://docs.oasis-open-projects.org/oslc-op/query/v3.0/os/oslc-query.html#oslc.where for details of the oslc.where query expression syntax.

The OSLC query expression syntax goes back more than 10 years and was designed to be a lightweight mechanism that would not be too onerous to implement regardless of how an OSLC server persisted data. It was designed to be a persistence-agnostic query syntax. For example, it doesn't support OR conditions, paranethetical clauses, the ability to test for property existence or non-existence, or use constructs like regular expressions for string conditions that might be expected in more extensive query languages like SQL, SPARQL, Cypher, Gremlin etc.

Thank you very much again for the response. It really helped a lot. 

You can achieve this by including the attribute in the oslc.select list and then post-processing the query results. My OSS ELM-Python-Client has an example oslcquery commandline app which does exactly this filtering to, for example, filter out (or in) just core artifacts - core artifacts have a folder URI in rm_nav:parent where module artifacts don't have rm_nav:parent.