One-half notrump is a treatment to show balanced hands below a strong 1NT opening. It is done by opening one of a suit, then rebid the lowest level of NT. It originally showed 13-15 HCPs when 1NT opening was 16-18 HCPs, however, this treatment becomes showing 12-14 HCPs when the 1NT opening is adjusted downwards to 15-17 HCPs.
If playing the weak notrump (12-14 HCPs), this treatment shows the exact opposite, i.e. 15-17 HCPs.