Red Green Texture Compression
There are 4 RG compression formats: unsigned red, signed red, unsigned red/green and signed red/green.
All of these basically work the same. They use compression identical to the alpha form of DXT5. So for red only formats, you have 1 64-bit block of the format used for DXT5's alpha; this represents the red color. For red/green formats you have 2 64-bit blocks, so that red and green can vary independently of one another.
The signed formats are almost identical. The two control colors are simply considered to be two's compliment 8-bit signed integers rather than unsigned integers, and all arithmetic is signed rather than unsigned. Also, rather than having 0.0 and 1.0 in the table, you have -1.0 and 1.0. Also, the comparison between alpha0 and alpha1 (red and green, depending on which block it is) is undefined if alpha0 is -127 and alpha1 is -128. That is because the comparison logic for two's compliment integers is slightly different from unsigned integers when comparing -128 to -127. The specification says that the result is undefined, so the resulting color values will be undefined.
Therefore, when compressing your signed texture formats, make sure you do not have a block that makes the first color -127 and the second -128.