Your mortgage adviser will go through a fact finding process with you. This is to understand your financial needs now and in the future and they will also discuss your current income and expenditure, along with any changes that you may foresee. They will then research and evidence the market, after which they will make a recommendation as to which product would be best suited to your circumstances.
You should ask for an explanation if you don’t understand any part of your mortgage illustration, including the terms and conditions.
If, after talking to your broker you are still in any doubt, you should seek independent legal and/or financial advice.