From the output of the "Get-Help Update-NeSnapshotVolumeConvertReadOnly -Full" command:
-RepositoryCandidate [<ConcatVolumeCandidate>]
Allows a repository candidate to be manually specified for use in the conversion. By default, the best candidate will be selected.
Required? false
Position? Named
Default value
Accept pipeline input?
Accept wildcard characters?
It's not a required parameter for the cmdlet, so unless you want to manually specify a repository candidate, it isn't needed.