Filters that are available for selection as a sub filter are related to the parent filter. In some instances, there will be several levels of sub filters available. When this occurs, the top level sub filter option will be expandable, denoted by an arrow. The following is an example:
This allows you to easily configure several levels of sub filters that will show objects related to the previous sub filter level. If you select a sub filter from an expanded list, the filter and the filter(s) that are levels above it will be added as sub filters. For example, from the example above, if you select Related Issues from the Open Issues filter expansion, both Open Issues and Related Issues will be added as sub filters. Open Issues will be the first level sub filter. Related Issues will be the second level sub filter.