Explain what usually decides which class of trading algorithm is chosen

The main classes are schedule-based algorithms (VWAP, TWAP and percentage of volume), arrival price or implementation shortfall algorithms, liquidity-seeking algorithms that search lit and dark venues, and algorithms that target the close. Selection depends on order size relative to volume, urgency, the security's liquidity and spread, volatility and the trade's benchmark. Schedule-based algorithms suit large, low-urgency orders in liquid names benchmarked to VWAP. Arrival price algorithms front-load execution for urgent orders or where price may move away. Liquidity-seeking algorithms suit large orders in illiquid names, taking liquidity opportunistically. Close-targeting algorithms suit index funds. The trap is ignoring that a percentage-of-volume algorithm leaves the completion time uncertain and can trade more when momentum is against the order.