Dismantling joints are normally used where the pipeline is mounted in such a way that it can't be easily pulled apart but you need to get access to a particular piece of equipment. For example if you had a flange mounted flow meter in a long straight piece of DI pipework it can be very difficult to remove or replace that flow meter. So you might put a dismantling joint on one side of the flow meter which will allow you to remove the flow meter relatively easily.
Flange adapter and coupling and mean different things in different countries. Typically a flange adapter i would take to mean as a conversion from a rubber ring or solvent weld type fitting to a flange of the same pipe size. A coupling typically means a female to female or socket to socket rubber ring or solvent weld pipe joiner.
If you asking whether these items can be interchanged or do the same job, the answer is no.
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"