It means just a connection, not a stopover (i.e. less than 24h.) In the case you cited, one of the (obscure) rules is being followed - two entries are allowed into Europe when one is just a connection without stopover to/from another continent en route to/from one of several countries in Africa whose only Oneworld service is to/from LHR. The other countries are Nigeria, Tanzania, Uganda and Kenya.

"Ex" means something different - it means originating in the "Ex-" country, viz. "My DONE4 is ex-ICN."