Hello,
Have only working knowledge with SLD / that too from the ESS perspective though :
System Landscape Directory (SLD), as per my understanding
Manages information about all installable elements of your system landscape.
As you must be knowing, SLD data suppliers connect to the SLD using :
1) ABAP Data Supplier service (Viewed using TCode : RZ70)
2) Java Data Supplier service
3) sldreg (Uploads XML data for standalone components, such as TRex)
For more read : System Landscape Directory - SAP NetWeaver by Key Capability - SAP Library
Further answering your Query :
For Central SLD :
One SLD is used for the entire Landscape, where multiple systems are connected.
Separate / Local SLD
are maintained to protect systems such as Production system.
For more, check out : System Landscape Directory / Planning Guide
Explains in detail the difference between Local and Central SLD.
Hope this helps.
Cheers,
Remi