What are "sending roadway along goaf" and "retaining roadway along goaf"?

Gob-side Entry Retaining —— With the advance of the upper section, the transportation roadway of the upper section is maintained in the goaf with special supporting materials as the return air roadway of the lower section coal mining face. Driving along goaf means driving along goaf, and attention should be paid to the maintenance of roadway. Due to the influence of secondary mining, support is a big problem. Driving roadway along the edge of goaf adjacent to the working face, leaving 2~5m clean coal pillar to continue driving. It should be noted that the mined-out area data should be accurate, especially the water pump pit, winch pit and gangue pit. , to prevent unprepared excavation. The low-lying water accumulation area near the airport should be drained in advance.