people really don't get that disposable income is money after compulsory taxes which is why it is after income tax, council tax (really take home pay - council tax)
as what you spend on food utilities housing childcare transport is within your control though you need these things you can choose between areas, size etc of housing whether you use waitrose or Aldi, whether you have basic phone or iphone10, how much your heating is on, whether you walk and take bus or have a car, and even though these are needs you have to cut cloth to suit if your disposable income is the average for UK about £2100 per month per household all the above must be less than £2100 this is the just about managing category there is little or nothing left for emergencies or discretionary spending like presents toys new clothes even prescription charges
what most people are actually meaning is discretionary spending (things you don't need but want) ie do you spend any spare money on school fees, clubs, dentistry, music holidays presents, eating out, takeaways, clothes hairdressers etc
some people have no discretionary spending because they are truly skint ie they couldn't spend less on food without being always hungry some have no discretionary spending because they spend more than absolutely necessary on needs ie bigger food bills, nicer phones larger houses because they would rather have a nicer house than a holiday etc this group is not skint