Meaning of -1 for scene boundary label

In the first 10 lines at the label file of `tt0047396.txt`, there is -1.
I think both 1 and -1 mean boundary.
So, I wonder why do you use -1 and what is the difference between 1 and -1?

```
0000 0
0001 0
0002 0
0003 1
0004 -1
0005 -1
0006 0
0007 1
0008 0
0009 1
```