LibraryResources like Kernel is a magic folder name. This old but useful video by Adam Berry uses this terminology. Using "GeneralUtilities`" on PacletInstall and following the trail will confirm my answer. One reason for including extensions meta data seems to be ensure compatibility of paclets with different versions of Mathematica. The notebook includes details of my spelunking. $basePathMap is very instructive summary of applications related to the paclet system and It shows some of the different extensions in use.

PacletManager`Package`$defaultLibraryRoot

Dataset /@ ((Flatten@*Values@
     PacletManager`Manager`Private`$basePathMap) /. 
   Paclet[x__] :> <|x|>)

This is a list of extensions in use.

{"Application", "AutoCompletionData", "ChannelFramework", 
"Documentation", "FrontEnd", "JLink", "Kernel", "LibraryLink", 
"Path", "Resource"}

Which can be obtained by

extensionslist = 
 Query[Flatten /* Union, "Extensions", All, 1]@((Flatten@*Values@
      PacletManager`Manager`Private`$basePathMap) /. 
    Paclet[x__] :> <|x|>)

Attachments:

Thank you for the comments! This isn't exactly what I was asking for.

You are showing how to create a paclet using the Workbench.

What I am asking is what are the valid extensions in PacletInfo.m, and what are their effects. The Documentation extension is documented in the Workbench help, so there's no difficulty with that one. But what effect does e.g. adding the LibraryLink extension have, as you can see in GitLink's PacletInfo.m? I can of course live without this information, but I am wondering what I can achieve with it.

I believe that your question can be answered by looking at GitLink: it shows how to package up a paclet, how to install it, and how to make it loadable with Needs. All this can be done without the Workbench. If you like, I'll post step-by-step instructions tomorrow. I'm fine with this part, I was simply wondering what different settings I can change in PacletInfo.m.