A short breakdown of how Midi is encoded with in an RTP message (After the header). For further reading see RFC 6295 3.2

Midi List Header & Midi List

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |B|J|Z|P|LEN... |  MIDI list ...                                |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| B | and | LEN |

B denotes how large entire header is.

When B is set 0 the header size is 8 bits long. In this mode LEN is 4 bits long & the MIDI List can be up to 15 bytes long.

When B is set 1 the header size is 16 bits long. In this mode LEN is 12 bits long & the MIDI List can be up to 4095 bytes long.

LEN denotes how long the Midi List is in bytes. LEN can be either 4 bits or 12 bits long depending on the state of B.

| J |

J denotes if a journal should be included.

When J is set to 1 there must be a Journal after each Midi command in the MIDI List.

When J is set to 0 the it means that there should be no Journal in the MIDI List section.

| Z |

Z denotes if the midi list includes 4 byte timestamps before each Midi command in the MIDI List.

If Z is 1 then timestamps proceed each Midi command.

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Delta Time 0     (1-4 octets long, or 0 octets if Z = 0)     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  MIDI Command 0   (1 or more octets long)                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Delta Time 1     (1-4 octets long)                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  MIDI Command 1   (1 or more octets long)                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                              ...                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Delta Time N     (1-4 octets long)                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  MIDI Command N   (0 or more octets long)                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If Z is 0 then there are no timestamps in the MIDI List section.

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  MIDI Command 0   (1 or more octets long)                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  MIDI Command 1   (1 or more octets long)                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                              ...                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  MIDI Command N   (0 or more octets long)                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| P |

TLDR use 0. You'll only need to consider using 1 if you are re-encoding Midi data that was originally transported over 5 Pin DIN or 1/4 inch jacks.

The short long version:

P denotes if the source used to generate the Midi List used the first status byte for all subsequent Midi commands. When P is set to 1 it denotes this original encoding. When set to 0 every Midi command in the source data included a status byte.

The P bit describes the MIDI source data stream, not the MIDI RTP list encoding; regardless of the state of the P bit, the MIDI list MUST include the status octet.

For further reading see the oping paragraphs of section 3.2

Midi List - Repeat Commands

A command status can be repeated without being specified. When a data value is in a place a status value should be the previous status should be used. A command status always starts with a 1 bit where as a data value always starts with a 0 bit. This difference allows us to detect the difference between a status byte and data value byte

90 3C 64 | 3E 64 | 40 46
|  |  |    |  |    |  |
|  Data    Data    Data
Status

When this mode is encoded into RTP MIDI the status byte is added before each data set.

90 3C 64 | 90 3E 64 | 90 40 46
|  |  |    |   |  |   |  |  |
|  Data    |   Data   |  Data
Status     Status     Status