For FS2004:
If by reflectivity you mean specularity (sun glare), then yes, it is set in the model for each material and will be displayed regardless of the texture suffix.
If you use an _T texture name, then the alpha must be white to avoid transparency. (Pure black is 100% transparent; pure white is 100% opaque.)
If you do not use the _T, and the alpha is white, then you get a flat (non-glossy) surface and no transparency, with specularity if specified in the model.
The darker the alpha gets, the more shiney and chrome-like it will get, and with specularity if specified in the model.
If you wish to have degrees of glossiness, you must use the _T name and use the _L texture to control the transparency and night time lighting.
This allows you to apply darker _T alpha areas to your paint scheme for glossiness while avoiding transparency.
You can use the _L to give you light effects at night.
The _L is simply an additive amount of light to the "lightness" already in the _T texture (applied only when aircraft lights are on).
These rules are part of Makemdl, the compiler, not gmax or FSDS.
So, to sum up:
1. If you want specular lighting, set the Specularity Level in material properties in the design program; Works regardless of texture suffix;
2. If you want chrome, use a standard named texture without the _T is best. (Works with _T and _L but goes gray at night);
3. If you want transparency, use _T and the alpha darkness will determine the degree of transparency;
4. If you want glossiness, or shiney surfaces, use the _T and _L. The _T alpha sets glossiness level, the _L eliminates the transparency and gives you night lighting effects.
5. The _L texture can also be used to add cabin, cockpit, gauge backlighting, nacelle, wing, or fuselage lighting effects at night when nav lights (black alpha in _L) are turned on, or with landing lights (white alpha in _L texture).
Hope this helps.