The ShockBurst bitrate mode (nrf_esb_bitrate_t) NRF_ESB_BITRATE_1MBPS_BLE is described as '1 Mb radio mode using Bluetooth low energy radio parameters.' in the SDK documentation. I couldn't find any further references.
How does NRF_ESB_BITRATE_1MBPS_BLE differ from NRF_ESB_BITRATE_1MBPS?