Key parameter in SNMP item. Why?

[Miguel22]

Hello!

I am preparing new Zabbix templates and I am using Cisco IOS by SNMP from Zabbix 7.0 as a model.

I can't find a reason why the Cisco template uses key parameters for some SNMP items. For example:

Name: Cisco IOS: System description
Type: SNMP agent
Key: system.descr[sysDescr.0]
SNMP OID: 1.3.6.1.2.1.1.1.0

Key parameter sysDescr.0 is not used in OID itself, OID is complete.

Then, you have can see in a trigger:

Name: Cisco IOS: Operating system description has changed
Expression: last(/Cisco IOS by SNMP/system.sw.os[sysDescr.0],#1)<>last(/Cisco IOS by SNMP/system.sw.os[sysDescr.0],#2) and length(last(/Cisco IOS by SNMP/system.sw.os[sysDescr.0]))>0

Key parameters just make the whole expression longer.

You can see the same in official documentation of SNMP agent:

https://www.zabbix.com/documentation/current/en/manual/config/items/itemtypes/snmp

Name: Interface wlp3s0: Bits received
Type: SNMP agent
Key: net.if.in[IfHCInOctets.3]
SNMP OID: 1.3.6.1.2.1.31.1.1.1.6.3

No explanation why the key parameter IfHCInOctets.3 is used. The only note regarding item key is: "Enter the key as something meaningful."

Any suggestion would be appreciated.

Michal