DDS & BMP.....Need understanding....

I'm trying to understand why some plane's texture folders are different from others. I'm still fairly new at repainting. I successfully repainted one of Thomas Ruth's airbus models as all I had to do was find the "main" dds file etc. Since this was the first and only type of texture folder I did so far, I understand it well.
But some planes have bmp. only, some with dds only, and some with both bmp. + dds files in the texture folders where the parts of the plane is scattered into many files. Why do some use only bmp while some use only dds or both?
But some planes have bmp. only, some with dds only, and some with both bmp. + dds files in the texture folders where the parts of the plane is scattered into many files. Why do some use only bmp while some use only dds or both?