This holster is brilliant quality, extremely well made, very comfortable to use, works very well and should last at least a lifetime. I use one at work that has been knocking around for many many years, can't do without it, and like it so much I bought one of my own. My Felco nş2 secateurs fit and live in it permanently and it feels very natural to use when I'm gardening both at home or at work.
I don't know if other brands of secateurs fit, so try them out in a shop display holster first if you haven't got felco secateurs. It should be a snug fit so they don't fall out and the leather will ease very slightly over time to mold to you secateurs.
You can clip it on your belt or use the slots to thread a belt through it. I always use the clip.
The belt clip was too tight at first but I eased it slightly with the aid of a couple of big screwdrivers to open it up a bit and now it's perfect. You could just permanently thread it onto a leather belt and wear that over your clothing and many of my colleagues do that and have other holsters and tools attached to the same belt as well.
Assuming you have Felco secateurs, and if you're in doubt or put off by the price of this holster, don't be, it's the real deal, a good investment that will last at least your lifetime.
It's used by all RHS horticulturalists, so if it's good enough for them...