This may be a simple question, but how do airplanes know which ACARS messages are meant for them from the ground? I'm not sure how physically close planes would have to be in order for that to be an issue. Do they simply receive all ACARS transmissions in the area and the CMU checks for this information in the header and continues to route it if it has the proper header? I'm assuming the VHF doesn't perform any checks since the data is still an RF signal at that point.
Every ACARS message includes the tail number of the aircraft it is to/from. Every aircraft decodes every message that it receives and then discards any that don’t match its own tail number. Since every tail number is unique, this is mostly sufficient.
The messages also contain a flight number, if applicable, which helps to distinguish a message about the current flight vs the last/next flight that aircraft may also operate that day.