Lucity caches certain data to make lookups faster and to reduce the load on the database server. This includes things like pick list values, field properties, as well as others.
The REST APIs are stateless web service applications. Restarting the app pool will not upset current users other than causing a very small delay the next time a request is made. However, restarting the app pool is not always realistic and is not possible directly from a client application.
It is possible to force a cache to clear on the REST API server from a client application using the $cache keyword and the DELETE method.
DELETE http://restapi.lucity.net/gbaMS/Work/WorkOrders.svc/$cache