The current *prop tables allow for tag-value pairs of an object (eg. organism), to have a free text value of a type given by a cvterm. However, in some cases, these values should be controlled vocabulary terms themselves.
For example, lets say we have a vocabulary that lets us describe the test and control of an expression analysis. Instead of adding an analysisprop that has the free text value "coronary artery", one could define the control as UBERON:0001621, which lets you hit the result when someone searches for "heart". In addition, if you wanted to add the GEO dbxrefs of the control samples as properties, you could associate those accessions while keeping it clear that these were auxiliary accessions associated with the analysis. (On a side note, there is no analysis_dbxref table in vanilla Chado).
Here are examples for the organismcvprop and organismdbxprop tables, adapted from the SQL for the organismprop table: