As Steven mentioned, only nodes, interfaces and volumes are important.
Other objects are not requiring additional licenses (i.e. CPU and Memory resources, routing tables, layer 2 and 3 topology, custom OIDs templates).
The NPM SL500 allows you to monitor 500 nodes (devices) and 500 interfaces and 500 volumes. This allows up to 1500 elements total but only 500 of each.
The NPM SL2000 allows you to monitor 2000 nodes, 2000 interfaces, and 2000 volumes. You can have up to 6000 elements but only 2000 of each element (node/interface/volume).
So if you only have 100 devices but want to monitor 750 interfaces, you will need to buy the SL2000 license (the SL500 will limit you to 500 interfaces).
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