An eminent member told me this could be a tip of the day, so I'll post it here.
This could seem like a basic thing you can find in the manual but i personally press these commands thousands of times in a campaign, and "drag boxing" can be a liability if you inadvertently do it twice or press [Ctrl] at the same time (leading to FF/Rally posts/Engineers leaving their intended position).
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"If you need to select more than all Military units in the planet, like MRS/Engineers/Scouts/Mobile Builders/Transport Ships/Force Fields etc, you have to [Ctrl+,] to select all Military Ships idle or not then [Shift + Left Click] on the Support Ships/Structures you need -on the Planet Summary- to add them to your selection (For instance, to select your forces and include Mobile Repair Stations).
On the other hand you can also [Ctrl+,] then [Alt + Left Click] on the Military Ships -on the Planet Summary- you want to remove from your selection (For instance when you want to separate your main force from specific ships you want to send after or will use for other purposes like Raid/Bomber/Siege Starships etc)."
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When used to it, it doesn't take more than 3 seconds of manipulation.