La D và applicata solo alle texture dinamiche,riporto quà la spiegazione (in inglese) tratta dall'API Guide,a pagina 31 del documento,paragrafo 1.12.3:
1.12.3 Defining dynamic textures
One way to provide information to the pilot in VC mode is by repainting the bitmaps used to
texture VC mesh groups. For example, you can implement gauges and data displays in this
way. You may even be able to re-use a panel area redraw method used for 2-D panels to update
a VC texture, minimising the additional coding effort.
Important: Orbiter can only draw into uncompressed textures. For this reason, textures which
support dynamic repainting must be marked in the mesh file with a 'D' (dynamic), e.g.
...
Textures 2
tex1.dds
tex2_dyn.dds D
Dynamic textures are less efficient than static ones, so you should try to keep them to a
minimum. Collect all parts that require dynamic updates in one or few (small) texture files, and
keep them apart from the static parts.
Indice 
Link 
