I am using my very old Husqvarna25 as example(it still goes strong)
The majority of brushcutters come with fittings for blades, even if they come with trimmer head there should be lock nuts and plates included to use blades.If you have not purchased one yet make sure you get all fittings in deal to fit blades.
My brushcutter had an extended shaft off the fitting you see here, this was to hold the trimmer head (that I soon threw away and replaced with an Ausdisc) In most brushcutters the extension will remove (see user/operator manual) sometimes it will be reverse thread so do not use excessive force or my favourite tool! (hammer)
This shows all you need to fit Powerweeder Pro
Powerweeder Pro , adaptor (if your fitting plate is 3/4" instead of 1") plate and lock nut.
If your plate/boss is 3/4" you place the small ring on the plate before fitting Weeder.
Note lock nut is not fully tightened , but it shows unit assembled.This model requires screw driver/rod to lock from rear then reverse thread tighten. Remember to tighten to undo !!