There are three operating modes for USB Device endpoints, including Auto-validation mode, Manual-validation mode, and Fl

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Auto-validation mode: After IN-token is received, data can only be sent when the data payload is equal to the Endpoint Maximum Packet Size (EPMPS). If a user needs to send a short packet of the remaining data in the end of data transmission, the user can set the SHORTTXEN of USBD_EPxRSPCTL[6] to 1. The controller will send data to the host in the next IN-token. This mode requires little CPU processing. Most of the processing is completed by the USB Device controller. This mode can be selected for use when the data payload sent to the host is always equal to the Endpoint Maximum Packet Size (EPMPS).

Manual-validation mode: Each time IN-token is received, CPU needs to write data size and number for data transfer. That is, this mode requires CPU processing each data transfer. If the number of data sent each time is not fixed, using this mode allows CPU to determine the number of data sent.
Fly mode: This mode is easy to use without any confirmation steps. Data will be transferred once In-token is received. If the data size exceeds the maximum packet size, the controller automatically packages it into a maximum packet size, and then sends the data with the maximum packet size to the host. This mode requires very little CPU processing and is suitable for synchronous data transmission because the transmission rate is more important than the packet size.