What do you mean by runways offered? If you mean during the select starting location then this is read via runway start locations in the AFCAD. If you open the T2G KSEA-AF.bgl file in ADE and select "lists"... "starts" you'll see only 16R/34L, and 16L/34R offered.  The middle runway simply has no start positions defined. 

 

The start points are the red circles:

 

Thanks Brian,

 

Duh on my part!!  I do see the lack of start points so it makes sense that the center runway isn't being offered by T2G-KSEA.  I don't know if the reason T2G isn't offering this option because the actual center runway has been undergoing resurfacing but that runway exists and the FAA currently offers ILS charts for the center runway and it clearly shows on the current airport diagram.  Why would T2G offer a substandard afcad for KSEA?  ORBX PNW's afcad does list the center runway.

 

I think I'll modify T2G's afcad and see how it works.  BTW T2G's latest release of their KSEA installer doesn't install the addon cleanly.  It leaves two blank entries at the top of the list in the scenery library.

 I wouldn't say the afcad is substandard because two start points were left out, it's an incredibly minor issue as most would just taxi to the active runway for departure. Either way adding one yourself is very easy to do if it's something you feel important to have included.

My T2G KSEA-AF.bgl doesn't have 16C/34C as start points either.

Did find this tho: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwj8k5ShzdLKAhVLw2MKHePyB5wQFggiMAE&url=https%3A%2F%2Fwww.portseattle.org%2FEnvironmental%2FNoise%2FDocuments%2Frunwayuseloaa.pdf&usg=AFQjCNH8JSR2QUUq5Rwp9O8Hr_AonQoRyg&sig2=uSyaUyNcS4KkZCdm_FolaA

 

I edited my afcad to use 16C/34C as the primary North/South departure runway, 16L/34R the primary arrival runway, and 16R/34L as the increased usage departure/arrival runway based on this (paraphrased from the above PDF): When airport demand increases, both 16L/34R and 16R/34R will be used for arrivals and 16C/34C will be used for departures. Is my case, my departure runways now have starts; 16L/34R no longer has start points.Works pretty good per that LOA (though editing the taxi paths to create good flow for AI was a bit fun also). P3D tends to go to the closet runway for takeoffs, so it generally works.