I believe that section 23 caters for rural/urban renewal schemes while section 50 is exclusively for student accomodation. the idea being to keep rents low by providing adequate supply/quality. while section 23 objectives are fairly self explanatory. this is a rather simplistic view and you would need to read up on it to be sure. im sure you will be able to get pleanty of info on oasis.gov.ie or revenue.ie