As described elsewhere, extensions are the mechanism by which the size of the core resources can be kept small. It is expected that any 'real' implementation of FHIR will require the use of Extensions - in other words using an extension is a normal part of FHIR.
As FHIR matures, it is anticipated that there will be a 'pool' of commonly used extensions for people to use, available in 'registries' that can be searched. In an ideal world, there shouldn't be duplication of extensions meaning the same thing - if you want to represent a persons ethnicity for example, it would be ideal if everyone uses the same extension to do so.
This is a 'work in progress', but the take away is that if you need to use an extension, first look to see if there is already one that meets your needs and use that. Only create a new one if there is no match, and ideally publish that extension to a registry so that others can find and use it.
Expect more work on this to come. You can see list of the extensions (and profiles) that are currently in the spec here, but be aware that that will change.
To find an extension in clinFHIR click the 'Find extension' button. The search dialog appears. There are a number of search parameters you can use, each with a description. To see what extensions others have created in clinFHIR then you can search for extensions where clinfhir is the publisher. The name and description are other good ones to use.
Once you have clicked search, clinFHIR will retrieve all matching extensions from the registry, and display them in a list.
Select the one you want and the click the 'Select button'.
