The Bosch external water filter part number 8703305356 replaces the existing smaller internal water filter screen inside certain Bosch tankless water heaters. The external water filter allows for easy access to clean the inlet screen periodically. Without the external water filter it is very difficult to get at the inlet screen inside the heater. The Bosch external water filter is designed to be used for Bosch models 250SX/2400E/2400ES/2700ES. The Bosch Integra 500/Evolution 500/2400ES/2700ES came with the external water filter.
All of the newer model Bosch tankless water heaters 830ES/940ES/Greentherm C950ES/Greentherm C1050ES/ C1210ES/C1210ESC have the built in inlet filter screen. See image below as it shows the inlet filter screen where the cold water connection is.