Why 1+1 = 2?¶
Disclaimer
This subchapter may or may not explain it 1:1 to the actual backend code logic. This subchapter only gives you the rough ideas on why things works.
Compression Card & Decompression Module is crucial for the network. It enables the network to do any kind of compression/decompression. Otherwise, your Bulk Cells is just a fancy infinite chest
Compressing/Up Crafting¶
It has been mentioned before that Bulk Cells can compress items automatically.
With the analogy of having to make patterns to craft nuggets to ingots, then ingots to blocks, then blocks to 1x blocks etc. this can adds up a lot in the pattern provider.
Instead, we can use Compression Card inside the bulk cells to enable ‘compression’. This will allow the network to automatically makes a ghost-pattern that handles all compression. Yes, everything. From as small as nuggets, up to 9x variants of compressed block (if possible).
Decompressing/Down Crafting¶
Borrowing the previous analogy, we also wanted to down craft those 1x blocks back into blocks and smaller. The bulk cells also handles down crafting quite similar to compressing. When a craft request a form of items at a smaller variant
Example, make 9 iron blocks but we only have 3x iron blocks
Then the system also makes a ghost-patterns that handles crafting 3x iron into 2x, 2x into 1x, and more until it reaches the desired amount from the system (9 iron blocks).
This is what I meant
If we requested 128 Iron Bars even though the system have enough ingots (in form of a compressed blocks), the system needs to convert it first into ingots from 3 5x iron blocks in order to use it. We do not have any pattern related to this compression inside the Pattern Provider. Hence, why we refer this to Ghost Patterns.
But WHEN it does the compression/decompression?
Generally it happens when the network interacts with item amounts.
This includes, but not limited to:
1. Items entering the network
2. The Network exporting out items
3. A Crafting Requests from an automation/player-requests for certain compressed items variants
4. You, the player, directly taking out/dumping items in the Terminal
for a regular player usage (it means Me, & YOU), all of this doesn’t really need to worry about. Don’t sweat it out :)
If you insert/extract items within the network, if it’s possible, it will always get unified with other compressed variants. And the system shows the highest compressed form possible in the network (here, it’s 1 9x iron & 3 5x iron) 
Regarding Energy Usage
It is also worth noting that every content (items/gas/fluid/etc.) movements inside an AE2 network requires energy (in the form of AE) and it’s relative to the amounts moved per tick. Compressing/Decompressing is no exception. On a rare case, One can stumble upon not enough network buffer to do any item compression, oftentimes resulting in a failed Crafting Request.
Applied Energistics 2 | CurseForge
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