The Application tag encodes the datatype and length.

+---+---+---+---+---+---+---+---+
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+---+---+---+---+---+---+---+---+
|   tag type    | c |   l/v/t   |
+---+---+---+---+---+---+---+---+

offset	meaning
0 bits 7 - 4	tag type
0 bit 3	class = 0 means application tag
0 bits 2 - 0	length, value, or type

Class
0 indicates an application tag. The tag number indicates the type.
1 indicates a context specific tag.

Sample Application tag:

C4 02 00 1B 90

C4 => 1100 0100
      tttt clll

field	data	value
tttt	tag kind	0xC = BACnetObjectIdentifier
c	class	0 = Application Tag
lll	length	4

02001B90	Object type(top 10 bits) = 8 (device)
02001B90	Instance Number(bottom 22 bits) = 7056

---

Tag Types

number	meaning
0	Null
1	Boolean
2	unsigned integer
3	signed integer
4	real
5	double
6	Octet String
7	Character String
8	Bit String
9	Enumerated
10	Date
11	Time
12	BACnetObjectIdentifier
13	reserved for ASHRAE
14
15

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See Also:
embedded systems - bacnet - Context Tag
embedded systems - bacnet - PD tags
embedded systems - bacnet - tag decoder