When the user taps a device to an NFC reader, the Android system needs to know which HCE service the NFC reader actually wants to talk to. This is where the ISO/IEC 7816-4 specification comes in: it defines a way to select applications, centered around an Application ID (AID). An AID consists of up to 16 bytes. If you are emulating cards for an existing NFC reader infrastructure, the AIDs that those readers are looking for are typically well-known and publicly registered (for example, the AIDs of payment networks such as Visa and MasterCard).
If you want to deploy new reader infrastructure for your own application, you will need to register your own AID(s). The registration procedure for AIDs is defined in the ISO/IEC 7816-5 specification. Google recommends registering an AID as per 7816-5 if you are deploying a HCE application for Android, as it will avoid collisions with other applications.