All Taxonator actions, searching by text, searching by code, navigating hierarchies, take place in the context of the currently selected search index. If you want to search for SNOMED terms, you would first select one of the SNOMED containing search indices from this dropdown. You may then navigate the taxonomic hierarchy up and down that is contained in that index.
If you select a different index and try to navigate the hierarchy, you will do so within the new index. If the currently selected term displayed in the middle panel is not contained in the newly selected index, you will not have much luck navigating the hierarchy. A SNOMED disease term probably will not be found in RXNorm so trying to navigate the trees to its children will not work as long as RXNorm is the selected search index
On the other hand, sometimes changing index is useful. If you search SNOMED active terms and select a disease, and then want to find related ICD10 terms, you can select the combined SNOMED & ICD10 index and navigate the hierarchy starting from the point of the SNOMED term you found. This can be a useful way to fill in values from other terminology systems
One of the powerful features of The Taxonator is the merged indices. These are combinations of different terminology systems based on the UMLS mappings between them. As noted above, this can be used to find SNOMED codes that are similar to a set of know ICD10 codes and vice versa. You can also use the combined NCI & RxNorm search index to find RxNorm drugs that fall under NCI categories. The regular RxNorm index uses ATC categories and hierarchy to categories drugs but that system has both strengths and weaknesses in that regard. Adding the NCI drug categories/hierachy can often help one complete a value set