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ensures that the last three numbers are used for indexing.

How this information looks like? 

The OID 1.3.6.1.4.1.2021.13.15.1.1.1 is defined as follows in the mibs: 

diskIOIndex OBJECT-TYPE
    SYNTAX      Integer32 (0..65535)
    MAX-ACCESS  read-only
    STATUS      current
    DESCRIPTION
        "Reference index for each observed device."
    ::= { diskIOEntry 1 }

Doing a snmpwalk

snmpwalk -c COMMUNITY -v 2c localhost 1.3.6.1.4.1.2021.13.15.1.1: 

1.3.6.1.4.1.2021.13.15.1.1.1.1 = INTEGER: 1
1.3.6.1.4.1.2021.13.15.1.1.1.2 = INTEGER: 2
1.3.6.1.4.1.2021.13.15.1.1.1.3 = INTEGER: 3
1.3.6.1.4.1.2021.13.15.1.1.2.1 = STRING: "fd0"
1.3.6.1.4.1.2021.13.15.1.1.2.2 = STRING: "sdb"
1.3.6.1.4.1.2021.13.15.1.1.2.3 = STRING: "sda"

So, in this case, 1.3.6.1.4.1.2021.13.15.1.1.1 is the index OID, and by default, it will use the last digit (By default) as the index, 1, 2 & 3 in the example: 

1.3.6.1.4.1.2021.13.15.1.1.1.1, 1.3.6.1.4.1.2021.13.15.1.1.1.2

rrd section

The rrd section defines what data will be collected and stored into rrd's.  Once again, the values defined inside the snmp section are like any other part of the model.  

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1. 'control' => 'CVAR=diskIODevice;$CVAR =~ /sda|sr|disk|xvda|dm\-/',
   This tells NMIS that the OID diskIODevice should be checked and only capture the values into RRD if they match the regular expression given. Please see the Advanced Modelling: When a single SNMP variable isn't enough page for further info  on custom variables.

2. indexed => 'true',
   Tell NMIS this is an indexed table, it will then go and use the index specified in the sys section above to iterate
3. graphtype => ''
   what graph-types will this rrd section create data for

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