What is the difference between P208 and P209 base runway materials?
From the FAA Airports Construction Standards Part 3 ('flexible base courses');

Just a side note that these are just little introductions to the materials. Download the word document from the linked page and just read that if you want.


  P208: This item shall consist of a base course composed of course aggregate bonded with fine aggregate base. It shall be constructed on a prepared subgrade or subbase course per these specifications and shall conform to the dimensions and typical cross-section shown on the plans.
  The aggregate base material shall consist of both fine and coarse aggregate.  Material shall be clean, sound, durable particles and fragments of stone or gravel, crushed stone, [   crushed slag,   ] or crushed gravel mixed or blended with sand, screenings, or other similar materials produced from approved sources. The aggregate shall be free from lumps of clay, organic matter, and other objectionable materials or coatings. 


And for P-209


  This item consists of a base course composed of crushed aggregate base constructed on a prepared course in accordance with these specifications and in conformity to the dimensions and typical cross-sections shown on the plans.
  Crushed aggregate shall consist of clean, sound, durable particles of crushed stone, crushed gravel, [   or crushed slag   ] and shall be free from coatings of clay, silt, organic material, or other objectionable materials. Aggregates shall contain no clay lumps or balls. Fine aggregate passing the No. 4 (4.75 mm) sieve shall consist of fines from the coarse aggregate crushing operation. If necessary, fine aggregate may be added to produce the correct gradation. The fine aggregate shall be produced by crushing stone, gravel, [   or slag   ] that meet the coarse aggregate requirements for wear and soundness.


You can download the Word file for more info on this, if you want to.