Possible classes to describe the function of PD externals:

• Control
  • Flow (bang, trigger, spigot ...)
  • Math (float, +-*/ == <= ...)
  • Lists (pack, unpack, glue ...)
  • Symbols (makesymbol, makefilename ...)
  • MIDI (notein, ctlin ...)
  • Data-structures (...)
  • Database (pool, qlist? textfile? ...)
• Signal
  • Flow (dac~, adc~, sig~. )
  • Generators (osc~, cos~, phasor~ ...)
  • Tables (tabread, tabwrite, soundfiler...)
  • Filters (env~, vcf~ ...)
  • FFT & Audio Math (fft~, clip~ ...)
• Graphics
  • Pixel Inputs (pdp_qt, pix_video)
  • Pixel Filters (pdp_convolve, pix_negative)
  • Pixel Outputs (pdp_xv, pix_write, pix_draw)
  • GemChains? (pdp2gem, translate, rotate ...)
  • Lighting (light, world_light)
  • Particle (part_*)
  • Vertex (vertex_*)

This first effort is multiple-level since I don't think a flat list of functional keywords will have a valuable granularity. No class list is perfect and things belong to any catagory only to some degree. Nevertheless such a structure breaks up the massive list of PD externals in a meaningful way to novice users. Please contribute!