Tarp Key -
it replaces the figure 9 for securing the SRL of a tarp. the single hole allows you to slide the key up and down the line for easy placement. the 'key' portion allows you to wrap the line rather than tie or twist around holes, cut outs etc. the wrapping of the line is what keeps the key located on the SRL and is meant to be on the same side or inline with your tarp. you then wrap around the tree, give a tug around the larger of the two anchor features to set tension, a couple of wraps around the anchor shape again. i finish with a couple twist loops that are pulled tight around those back two features.
i use two tarp keys but when dblcorona and i were going back and forth he mentioned he likes to use a fixed end, so...
Tarp Cross -
this one is a little more diversed. you can use it at both ends or just one end. if used at one end you simply create a fixed loop on your SRL and feed it through the single hole then up and over the hook pulling it tight over the 'T' shape which is the cross. when setting your tarp up you simply loop around the tree and hook the SRL. on the opposite side you can use a cross or a key. if using a cross you want that on the tail end of the SRL, so if you have the SRL pulled taut on your tarp (cause the other side is already set) you wrap the line around the tree with the loose line fed through the cross hole and you on the say the right side of the tree while the taut SRL is wrapped around on the left - confussing. you then hook the taut SRL, pull the loose tail of the SRL tight which should just be fed through the single hole of the cross right now and then finish with a couple wraps over the 'T' and two twist loops pulled tight.
the cross has a little more diversity. people have found you can use it as a quick release for pull outs on WBBB as i demostrated. dblcorona used it to fix the floopy side of a gathered end hammock.
i hope this clears up anything you were confused on. you can use them on any tarp even if it doesn't have a SRL. if you have a stock HH tarp it works just fine, only difference is you can't slide your tarp along your SRL if by chance you strung it up off center. both the key and the cross are designed to work with lines 2.2mm in diameter or smaller.