Subtraction mode determines the colour of the displayed pixel by subtracting the colour value of the selected object pixel from the colour value of the corresponding one on the layer below. On the whole it gives darker shades. This is one of the modes where it makes a difference which way round the objects are. If you reverse them, so that the roses are the background and the green ramp is the object, then subtracting the colour value of the object pixels from the underneath ones is not going to give you the same answer as the other way round did. Addition and difference on the other hand, will always give you the same result whichever of your images is the top one. The picture below shows the effect of subtraction mode when the green ramp is above the roses.

Multiple mode multiplies together the colour values of the selected object pixels with the corresponding pixels and colours the displayed pixels in accordance with the result of the calculation. It gives the same answer whichever way round the layers are, so it does not matter which is the object and which the background here. The result will be the same. I find I use multiple a lot, because it seems to give a very good blend of two images, giving each of them equal weight and not distorting the colour of either. I use it a lot with a light neutral coloured texture and another image, to give the image texture without changing its colours.

Inverse of multiple makes exactly the same calculation as multiple does, but it then applies the inverse of the answer. The resultant blend is usually lighter than multiple, and again, it makes no difference which way round the layers are.

Those are all the blend modes. You are not limited to using just one object with one object or background below it. You can have as many layers as you like and blend them all differently. In this image I still have the greyscale ramp as the background, the roses as the object immediately above the ramp, and I have added above that, the file from PhotoImpact textures called Leaf4.jpg (reduced in size to 200 pixels high). The roses merge mode is lighting, and the leaf merge mode is difference. The result is on the right. I will leave you to play around with these modes and try different combinations. I want now to show you one or two practical uses for object merging.