1) Low-Frequency System: With 125KHz frequency chip, work with T5557 card. 2) High-Frequency System: With 13.56MHz frequency chip, work with Mifare 1K card. Temic 5557 card: low-frequency card (125KHz), small storage space, short reading distance, mainly used for opening doors. Mifare 1k card: High-frequency card (13.56MHz), bigger storage space, longer reading distance, more sectors and multifunctional. ※ If the key card is used only for opening door, T5557 card system will be enough. ※ If the key card is for mutifunctional purpose instead of just opening door, such as for consumption system, parking system, access control system and so on, you need to choose Mifare 1K card system. PS: High-Frequency hotel lock system is US$5 higher than Low-Frequency hotel lock system. Appearance is the same.