It's on the label as mm diameter, doesn't follow the norm of core/wrap unless I've read it wrong & it's 38AWG core/28AWG wrap which is possible. Staples use wrapped cores with an outer wrap but checking a roll of staple there's no way it's a 28AWG outer wrap, more like 38/40AWG. I don't pay much attention to the composition other than wire type, as long as there's ohms/length it's less stressful to use that. Or go really old school & make a coil & measure on a reader/mod & add/remove wraps as required.