These column values are maintained unique in order to provide the user with convenient ways to select either the prime source or (via the selection expression parameters of rgsregions) a subset of sources for which spectrum-extraction regions are desired. When a previously-made RGS source list is modified, the task checks to make sure the labels and index values in this list are unique. (They are left so by any previous invocation of rgssources, but values might be changed in the interim by use of the appropriate ftool, for example.)
Further added sources from either a submitted EPIC list or the user are checked one by one against all previous LABEL values in the list before themselves being added to it. An INDEX value equal to the next highest unused value is given to each added source.
If a source is found to have the same LABEL value as an existing member of the source list, the action taken by the task depends on the value of the parameter clobberonlabel. If clobberonlabel=no (the default), the task fails with an error. Otherwise it discards the newer source, with a warning.
