the meaning of 0 in the dataset of "edges"

[Yovecent]

what's the meaning of 0 in the dataset of "edges"? Does it mean the outside?

#first four entry are per list are rooms edges and 4th and 6th are showing what room type is on each side of edge 
"edges":[72.0, 161.0, 72.0, 220.0, 3, 0], ...,[107.0, 130.0, 72.0, 130.0, 4, 0], [148.0, 28.0, 148.0, 87.0, 1, 2]] 

#room indexes that are on each side of the edges
"ed_rm":[0], [0], [0], [0, 2], ..., [2], [2, 3], [2, 1], [2, 0], [2]]