how to use the joern-slice tool?

[lyqssb]

usage: joern-slice [-h] [-f [FILE]] [-o [OUT]] [-c] [-g]
[-a ATTRIBUTES [ATTRIBUTES ...]] [-s {forward,backwards}]
[-l LABEL] [-k ORDER]

Extracts program slices.

optional arguments:
-h, --help show this help message and exit
-f [FILE], --file [FILE]
read input from the provided file
-o [OUT], --out [OUT]
write output to provided file
-c, --complete Output the complete node, not just its ID.
-g, --gremlin query is a gremlin traversal as opposed to a lucene
query
-a ATTRIBUTES [ATTRIBUTES ...], --attributes ATTRIBUTES [ATTRIBUTES ...]
Attributes of interest
-s {forward,backwards}, --slice-direction {forward,backwards}
The slicing direction
-l LABEL, --label LABEL
Traversal for labeling
-k ORDER, --order ORDER
The number of hops to follow in the PDG
I don't know the mean of PDG ,there is no command to get a PDG of program. How can i get a PDG?