There are two merge in the game: the merge of the main expedition and the temporary expedition merge.
To find out if you are on a main or temporary expedition, look in the lower-left corner of the screen. If a map is displayed there, you are on a main expedition. If there is an arrow, you are on temporary one.
You are on a main expedition: 
You are on a temporary expedition: 
The button that opens the merge field is located in the lower-right corner of the screen. It looks like this:
What is the difference?
The Merge of a main expedition:
In the merge of the main expedition, you complete tasks, receiving energy and new generators as a reward.
The Merge of a temporary expedition:
In the merge of a temporary expedition, you get points for completing orders. The more points, the more rewards there are in the Gift Fair 
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