Every route into an interlocking must have a governing signal. Signalled sidings signalled for travel toward the interlocking. Yard tracks, industrial spurs, etc. On some railroads, at some locations, for any low-speed route into the interlocking, even for main tracks. Railroads tend to install high signals at the entrance to an interlocking to govern those tracks that allow a high speed route into the interlocking.
Obvious candidates are main tracks and signalled sidings.
However, not all main tracks qualify - see below. Yard tracks, unsignalled sidings, and industrial spurs entering an interlocking did not allow a speed to warrant a high signal. In most cases, such unidirectionally signalled tracks warranted only a dwarf signal. In those cases where no routes into an interlocking were 'high speed routes', practice varied from railroad to railroad. Other railroads installed dwarf signals at such locations, since the low speed did not require longer sighting distances.