There are 2 sizes of needle. Bigger one is 0.8mm and the smaller one is 0.6mm
To use the needle and holder you must put the small needle in the correct hole at the end of the holder and/or the bigger needle inside the correct hole at the other end of the holder, If you try to put the bigger needle inside the hole for the smaller needle it will not fit in.
To put the needle in, unscrew the top/end of the holder without removing the screw entirely, then put the needle into the visible + hole and then screw back, If you have used wig ventilating needles before then you won’t have a problem setting this up as it is done exactly the same way.
With this needle and holder, you have the advantage of having 2 different sized needles attached to the same holder