ESET product modules have two types of updates, Level updates and Nano updates. They differ in time range.
There are 3 Level Updates - Level 0 (L0), Level 1 (L1) and Level 2 (L2). There are 20 Nano updates, from Nano 1 to Nano 20. 
Nano updates represent the number of updates from the very last time of latest update, meaning, Nano 1 represents the very last update (for example updating virus signature database from version 5001 to 5002), Nano 20 represents the last 20 updates altogether. Upon next available update, all Nano updates are regenerated (Nano1 becomes the latest update, former content of Nano1 is drained into Nano2, etc.). 
A program module (for example HIPS module) is updated with just one out of all Nano updates available. See the example below. The number of Nano updates issued per specific module might differ from module to module and within the time.
Level updates represent the number of updates from the release date of particular program module. L0 includes several updates from the time the particular module was released.  L1 includes some subsequent updates and L2 the remaining updates, including all 20 Nano updates.
Once L2 approaches the size of L1, updates included by L2 are drained into L1. Once L1 becomes as large as L0, updates included by L1 are moved to L0.
L2 is gradually filled  by older Nano updates, meaning, content of Nano 20 is moved to L2 when all Nano updates are regenerated upon a new available update.
After a fresh new installation of a product, the product is in its basic state and receives all Level updates:
Level Updates
Basic state Up-to-date state
If a product is updated on regular bases, but has been offline for a few days, and after connecting to the update server Nano 3 update is identified as the missing one, then it will be applied:
Applying Nano update
Basic state Actual state Up-to-date state
An example of update files of a 32-bit engine module
Level updates Size (kB)
Level 0 32 212
Level 1 14 352
Level 2 504
Nano updates Size (kB)
Nano 20 200
Nano 19 196
Nano 18 189
Nano 10 147
Nano 3 31
Nano 2 28
Nano 1 21