The host operating system is completely out of the picture: the traffic goes direct from the NFC antenna to/from the SE, without traversing the Android path at all. Specifically for Android card-emulation involves means routing communication from an external NFC reader directly to the embedded secure element, which can appear either as 14443 contactless smart-card or Mifare classic tag.

In this mode the phone emulates an NFC tag. (There is also an optimization to register a callback to create that NDEF record on demand, without committing to it in advance.) On the recipient side, Beam is handled directly by the NFC service by invoking the right application. The stars have to align for this, with another device in RF range of the phone and transfer is confirmed by the user by clicking on the screen at the right instance. Peer-to-peer, the basis for Android Beam. It is not possible to directly use this mode via Android API either on the sender or recipient side. Instead applications can declare an NDEF record to be transmitted if a beam-transfer is initiated.

Examples include scanning a URL from a tag (such as Pay-By-Phone stickers on parking meters in SF, or reading information from a US passport) The Android NFC stack provides extensive support for this mode, in callback model: applications can register to receive notifications on discovery of tags, either by NDEF or tag type– such as Mifare classic tags or all ISO 14443 smartcards. This is probably the most common scenario. The host device functions as the active participant, while on the other side is a passive tag that powered by the induction field generated by the phone. Quick note about the different modes for NFC usage supported in Android:

Mifare card emulation on android