|-7-|-6-|-5-|-4-|-3-|-2-|-1-|-0-|
    |---+---+---+---+---+---+---+---|
 1: | PDU-type      | s | m | a | 0 |
    |---+---+---+---+---+---+---+---|
 2: | 0 | max seg   | max APDU      |
    |---+---+---+---+---+---+---+---|
 3: | invoke id                     |
    |---+---+---+---+---+---+---+---|

fields
bits	name	data type	description
byte 1 bits 7 - 4	PDU Type	INTEGER	See table below
byte 1 bit 3	s	BOOLEAN	this is a segmented message
byte 1 bit 2	m	BOOLEAN	TRUE if this is a segmented message, and this is not the final segment
byte 1 bit 1	a	BOOLEAN	TRUE if the requester will accept a segmented BACnet-Complex-ACK-PDU as a response
byte 1 bit 0	0	0	not used, must be 0
byte 2 bit 7	0	0	not used, must be 0
byte 2 bits 6 - 4	max seg	INTEGER	this parameter specifies the maximum number of segments that the device will accept.
byte 2 bits 3 - 0	max APDU	INTEGER	maximum size of a single APDU (Application Protocol Data Unit) that can be accepted by the device.
byte 3 bits 7 - 0	invoke id	integer	used to match requests and responses

PDU-Type
0x0X	BACnet-Confirmed-Request-PDU
0x1X	BACnet-Unconfirmed-Request-PDU
0x2X	embedded systems - bacnet - BACnet-SimpleACK-PDU
0x3X	BACnet-ComplexACK-PDU
0x4X	Segment ACK
0x5X	Error-PDU
0x6X	Reject-PDU
0x7X	Abort PDU
0x8X - 0xFX	reserved

found this without explanation:

0x02	APDU type 0: Confirmed Request; Bit1: Segmented Response Accepted