The CodeSystem http://www.hl7.org/fhir/codesystem-expansion-processing-rule.html is the basis for the ValueSet http://www.hl7.org/fhir/valueset-expansion-processing-rule.html used by the extension http://www.hl7.org/fhir/codesystem-expansion-processing-rule.html defining how concepts are processed into the expansion when it's for UI presentation.

Currently three different codes are possible all-codes, ungrouped, and groups-only. However, the description of the three codes is not clear.

• What is the difference between all-codes and ungrouped? The first includes "all codes and any defined groups" while the latter "lists the groups, and then any codes that have not been included in a group". As a result, in both cases all codes would be included.
  • Reading only the name of the code ungrouped one could assume that only those codes are included in the expansion which are not part of a group.

Furthermore, a reference to the  http://www.hl7.org/fhir/valueset-operation-expand.html operation and its parameter excludeNotForUI (respectively, forUI according to FHIR-30700) would help to understand what "(when in UI mode)" means.