The Normal Walsh version is required, the LDP is optional but recommended.
The Document Style Semantics and Specification Language tells jade (see Section 3.3) how to render a DocBook document into print or online form. The DSSSL is what converts a title tag into an <H1> tag in HTML, or to 14 point bold Times Roman for RTF, for example. Documentation for DSSSL is located at the same site. Note that modifying the DSSSL doesn't modify DocBook itself. It merely changes the way the rendered text looks. The LDP uses a modified DSSSL (see below).
The LDP DSSSL requires the Norman Walsh version (see above) but is a slightly modified DSSSL to provide things like a table of contents.