The three buttons let you to specify whether the Alpha channel in your Color Map is used as a Reflectivity mask, a Clip mask or as an Emissive mask.
By default, Lumion will set the second button - Color Map Alpha Clips Object - to On.
Please note that the Standard material only supports a 1-bit Clip mask, so each pixel is either fully transparent (black) or fully opaque (white). If your Clip mask contains greyscale tones, they will be rounded up/down to pure black/white. Reflectivity masks support 8 bits, i.e. all 256 greyscale tones in the alpha channel.