If you monitor any packet radio frequency you may see strange letters at the embedded in the traffic. Additional information can be found here

Here’s an example:

1:Fm N4SER-1 To W2DEN <RR P/F R2 >[08:52:54]
1:Fm N4SER-1 To W2DEN <DISC P>[08:52:54]
1:Fm W2DEN To N4SER-1 <UA F >[08:52:54]
1:Fm N4SER To ID <UI pid=F0 Len=18 >[08:53:59]

Fm The station sending the packet

To The station to which the packet is addressed

<  > The header information that was transmitted.

<xxx xxx are ‘Control Codes’ explained below:

Unnumbered frames:
SABM (Set Asynchronus Balanced Mode)
The SABM command is used to place 2 DXEs in the asynchronous balanced mode. This is a balanced mode of operation known as LAPB where both devices are treated as equals.Information fields are not allowed in SABM commands. Any outstanding I frames left when the SABM command is issued will remain unacknowledged.The DXE confirms reception and acceptance of a SABM command by sending a UA response frame at the earliest opportunity. If the DXE is not capable of accepting a SABM command, it should respond with a DM frame if possible.
DISC The DISC command is used to terminate a link session between two stations. No information field is permitted in a DISC command frame.Prior to acting on the DISC frame, the receiving DXE confirms acceptance of the DISC by issuing a UA response frame at its earliest opportunity. The DXE sending the DISC enters the disconnected state when it receives the UA response.Any unacknowledged I frames left when this command is acted upon will remain unacknowledged.
FRMR (FRaMe Reject)
The FRMR response frame is sent to report that the receiver of a frame cannot successfully process that frame and that the error condition is not correctable by sending the offending frame again.
UA (Unnumbered Acknowledge)
The UA response frame is sent to acknowledge the reception and acceptance of a SABM or DISC command frame. A received command is not actually processed until the UA response frame is sent. Information fields are not permitted in a UA frame.
DM The disconnected mode response is sent whenever a DXE receives a frame other than a SABM or UI frame while in a disconnected mode. It is also sent to request a set mode command, or to indicate it cannot accept a connection at the moment. The DM response does not have an information field.Whenever a SABM frame is a received, and it is determined that a connection is not possible, a DM frame shall be sent. This will indicate that the called station cannot accept a connection at that time.While a DXE is in the disconnected mode, it will respond to any command other than a SABM or UI frame with a DM response with the P/F bit set to 1.
UI (Unnumbered Information)
The Unnumbered Information frame contains PID and information fields and is used to pass information along the link outside the normal information controls. This allows information fields to go back and forth on the link bypassing flow control. Since these frames are not acknowledgeable, if one gets obliterated, there is no way to recover it. A received UI frame with the P bit set shall cause a response to be transmitted. This response shall be a DM frame when in the disconnected state or a RR (or RNR, if appropriate) frame in the information transfer state.
Numbered frames:
RR (Receive Ready)
Receive Ready is used to do the following:

  1. to indicate that the sender of the RR is now able to receive more I frames.
  2. to acknowledge properly received I frames up to, and including N(R)-1, and
  3. to clear a previously set busy condition created by an RNR command having been sent.

The status of the DXE at the other end of the link can be requested by sending a RR command frame with the P-bit set to one.

RNR (Receiver Not Ready)
Receive Not Ready is used to indicate to the sender of I frames that the receiving DXE is temporarily busy and cannot accept any more I frames. Frames up to N(R)-1 are acknowledged. Any I frames numbered N(R) and higher that might have been caught between states and not acknowledged when the RNR command was sent are not acknowledged.The RNR condition can be cleared by the sending of a UA, RR, REJ, or SABM frame.The status of the DXE at the other end of the link can be requested by sending a RNR command frame with the P bit set to one.
REJ (REJect)
The reject frame is used to request retransmission of I frames starting with N(R). Any frames that were sent with a sequence number of N(R)-1 or less are acknowledged. Additional I frames may be appended to the retransmission of the N(R) frame if there are any.Only one reject frame condition is allowed in each direction at a time. The reject condition is cleared by the proper reception of I frames up to the I frame that caused the reject condition to be initiated.The status of the DXE at the other end of the link can be requested by sending a REJ command frame with the P bit set to one.