short answer: when paired directly to the ST hub, a Zigbee bulb will adopt the ZHA profile and will show on a Zigbee network map as a repeater as long as the bulb is on power. However, multiple community members have reported that these bulbs are inconsistent when acting as repeaters, and messages for devices of other device classes may be dropped or lost. So they may be able to help strengthen your network, but you should not rely on a bulb as the only ZHA repeater in a zone.