As 1 is occupied for the stronger hands from 16hp onwards, we have to combine hands with a longer minor suit into one opening bid : 1. An interesting side-effect of this approach is that there is always an 'escape' bid available : 2/3/4 , pass-correct.
As it is important to know as soon as possible whether a major suit in partners hand is a 4 or 5 card, we try to use the limited bidding space optimally for that purpose :
1 : from 6+hp onwards an unknown 4 card major. If 12+hp, any distribution without 5 card major is possible.
1 : <10hp , 5+ card
1NT : <10hp , 5+ card
The partnership knows that game is not intented.
Stronger hands with a 5
card major are bid in transfer : 2/2
Without a 5 card major, the partner can bid 2/3NT , or, with 12+hps, go via 1 .
Using transfers , 2 becomes avaialble to show a hand that else becomes difficult to bid : 54 , 10-11hp, NF.
The 3-level, 3, are there to show long suits.
WIthout a 4 card major, there is the escape 2 , P/C, (<10hp) and/or with 10-11 hp : 2NT , or with 12+hp : 3NT.