The main difference between manholes and handholes is simply the size of the hole. The term "manhole" refers to inspection holes installed on communication system pipeline or shafts, which can accommodate a person to pass through (either钻过去 or钻进去) during inspections or maintenance of such equipment. In contrast, handholes are smaller and only allow for hands to be inserted for inspection or operation. Additionally, both manholes and handholes are not only required in communication engineering but also in complex systems like HVAC (mainly central air conditioning) and power line systems, where such inspection openings are typically provided.