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This is the Almost Full Flag. This is the only mask you'll need to use the interrupt. It is used to enable the hardware to throw an interrupt when the FIFO hits the watermark level.
P_UART_INT_ENABLE |= P_UART_ISR_RX_AFF_MASK
Clear the bit:
Disable the interrupt:
P_UART_INT_ENABLE &= ~(P_UART_ISR_RX_AFF_MASK)
Set your watermark level to x:
The other flags you see defined are either not necessary to use or their use case is covered by the almost full flag e.g. rx completely full, rx almost empty, rx completely empty, etc.
Thank you, Jake!