This is player crafted and allows a Level 5 Settlement owner to 'Expand' his / her settlement by an addition 5 Slots. Now before you rush off I MUST point out that when added the expansion adds Five 'Greyed out' slots to your Settlement.
To unlock these slots you must invest SRP in the form of research. This expansion allows you to gain more slots for your settlement and 'prepare' for Level six.
There will be 'other add-ons coming in the next few weeks which will 'tempt' you down this path but ultimately its your choice.
I didn't go 'Mad' on the resources for the Expansion Unit but it does use a wide selection of commodities and Nebula items so don't expect it to be a walk in the park. Base construction value is in the mid 4-5 million credits. Player value WHO knows
Just one last thing: If you have multiple settlements and want to upgrade both don't fret. Once you have one (SEU) in your Misc bay (IE having crafted or purchased from a crafter) upon entering your settlement overview you will be asked if you want to deploy it. If you do not want to just choose no. You WILL be prompted each time you land at the over view page.
Coops
Have fun