Answer

The architecture and design of SNMP frameworks is defined in RFC 3411.

The snmpEngineID has a length of 12 octets.

                    The first four octets are set to the binary

                    equivalent of the agent's SNMP management

                    private enterprise number as assigned by the

                    Internet Assigned Numbers Authority (IANA).

                    For example, if Acme Networks has been assigned

                    { enterprises 696 }, the first four octets would

                    be assigned '000002b8'H.

                    The remaining eight octets are determined via

                    one or more enterprise-specific methods. Such

                    methods must be designed so as to maximize the

                    possibility that the value of this object will

                    be unique in the agent's administrative domain.

                    For example, it may be the IP address of the SNMP

                    entity, or the MAC address of one of the

                    interfaces, with each address suitably padded

                    with random octets.  If multiple methods are

                    defined, then it is recommended that the first

                    octet indicate the method being used and the

                    remaining octets be a function of the method.

The length of the octet string varies.

                    The first four octets are set to the binary

                    equivalent of the agent's SNMP management

                    private enterprise number as assigned by the

                    Internet Assigned Numbers Authority (IANA).

                    For example, if Acme Networks has been assigned

                    { enterprises 696 }, the first four octets would

                    be assigned '000002b8'H.

                    The very first bit is set to 1. For example, the

                    above value for Acme Networks now changes to be

                    '800002b8'H.

                    The fifth octet indicates how the rest (6th and

                    following octets) are formatted. The values for

                    the fifth octet are:

                      0     - reserved, unused.

                      1     - IPv4 address (4 octets)

                              lowest non-special IP address

                      2     - IPv6 address (16 octets)

                              lowest non-special IP address

                      3     - MAC address (6 octets)

                              lowest IEEE MAC address, canonical

                              order

                      4     - Text, administratively assigned

                              Maximum remaining length 27

                      5     - Octets, administratively assigned

                              Maximum remaining length 27

                      6-127 - reserved, unused

                    128-255 - as defined by the enterprise

                              Maximum remaining length 27 octets

Let’s take NNMi EngineID for example and dissect it:

80 00 00 0B 7F 9E 38 6E D7 C1 B4 49 71 AA CF 71 3B C6 20 C3 05

Following the logic that:  

1. The fourth octet (7F in Hex) should be the format defining octet.
2. Up until the fourth octet are enterprise identifiers.

We can determine the following:

1. The format of the ID is reserved / unset type based on the below table. 7F in HEX = 127 in Decimal
2. The ID cannot be modified in the first four octets, 7F is the format defining and 0B is part of the enterprise ID

0B in HEX is 11 in Decimal, which is the enterprise SNMP ID of Hewlett-Packard. Used by NNMi.

As we can see below:

Every single NNMi instance will have engine id with the same values and format, containing 80 00 00 0B 7F

The characters and length are not changeable in NNMi.

Some Cisco devices, for example, do not allow for EngineIDs bigger than 40 characters to be configured in their SNMP communication. 

They will not be able to communicate with NNMi, because its ID is bigger (42 characters), but it does comply to the regulations and standards (only 21 octets, with maximum allowed 31).