shell holder

When installing a die the instructions say to raise the ram to the stop and screw in the die until it touches the shell holder. Question: Do you fully seat the die to the shell holder so that it's snug or do you just touch the shell holder with the die? I've seen it done both ways in videos...

Can someone tell and show me the difference between the J and X type shell holders on a Lyman press? Ive looked on the net and cant seem to find anything. Thanks