Thanks SkullmanAlpha for answering Monkeyz743's question.

As the last paragraph int eh description says, "All ships, vehicles and trailers in this set have a rear mounted Merge block and can be printed". Here is the printer I use and test all of them with before publishing: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1967085548

Sometimes the merge blocks fits the design, and so I leave it, other times (like if there is a rear connector that it is blocking), I will paint it red and put instructions in the description for it to be ground down.

That is a merge block. This series of Spite's work are designed to be projected and printed. After it is welded the merge block is disconnected. In some of the blueprints (like this one) the merge block is kind of tacked onto the design and is intended to be cut off (these ones tend to be painted red). Other blueprints have the merge block designed as part of the ship/vehicle and remains.

Hello, been looking through all your creations and always seem to notice a red, docking port looking block. Can you tell me what this is as I have no idea.

If its from one of the DLC's (which I have none of) my bad for not knowing its just a stock part.