Part of the piping for the shop tools (table saw, router table, drill press, band saw, floor sweep and general use hookup port).
The main run from the dust collector is 6" solid pipe, split off to two 4" runs at the top right here (one for the tools shown here and one for the miter saw).
Manual blast gates are used to control flow (those red parts in-line with the piping at strategic locations).
The jointer run continues off from the top left.