SQL swap files are used for on-the-fly sorting of large query result sets. By nature, they are temporary, however, can unexpected consume more disk space than expected for some extremely large query results. For example, an ORDER BY clause may require a sort of a very large final result set that requires sending these to disk. This storage location can be specified in ctsrvr.cfg such that these files can be written to a volume of sufficient storage capacity.
These files usually show up as LTS_* and are removed when a query execution requiring additional storage space completes. After killing a SQL connection creating (owning) these files it may in some cases continue to completion before removing files.