What is the difference between Bottom Space to Bottom Layout Guide and Vertical Spacing to Bottom Layout Guide in Interface Builder?

Do you read the two in the same place, or are "Bottom Space to Bottom Layout Guide" and "Vertical Spacing to Bottom Layout Guide" 2 messages that appear in different popup or IB panes ?

I actually read Vertical Spacing to Bottom Layout Guide when I am told I should be seeing Bottom Space to Bottom Layout Guide. I have an image in IB that, when I Ctrl-drag from the image to the white space directly below it, the options that appear don't contain Bottom Space to Bottom Layout Guide. I see Vertical Spacing to Bottom Layout Guide instead.

I usually do not bother at this. I believe (?) it is just two different formulations depend on type of objects the constraints weighs on.