technically its a 50k lump sum retention bonus. but it operates as a 10k retention bonus every year after the 3rd year of employment. if you decide to leave the company before the 7 year period, any retention bonuses paid get taken away and the interest is recalculated for the higher amount.
at the end of the day, it's money making scheme any management will try their hardest to get as much money out of you regardless of what the contract stipulates. many businesses are guilty of this and it's up to the cadet to challenge the decisions by management.