If you click on the tag and read the text above the questions (and click read more) the tag wiki will answer your question. I've copied them here for reference. Putting only science-fiction on a question may mean that some answers are more suitable for soft-scifi, while science-based says that answers must be based on real science (or extraptolations from that). It is important to note that science fiction isn't always based on science. For instance star-trek, an iconic sci-fi show, can actually be classified as fantasy in space. None of it is rooted in science.
To give an example, your question on weaponising FTL is not an appropriate candidate for science-based as time travel contradicts the laws of physics as we know them and you seem to want to allow that based on your answer.
Bonus: hard-science goes one step further than science-based (and you don't need both) and specifies that only existing science can be used to answer the question (and you must cite references, show equations etc)
This tag should be used on questions relating to settings in science-fiction. This tag merely denotes the world's setting, and answerers should take this into account when coming up with answers: answers to science-fiction questions are also required to have the elements of science fiction in them that the question has.
Questions with this tag should be answered as far as possible based on known scientific fact or reasonable extrapolations from that. For example, we do not currently have the material science required to build a space elevator, but we can discuss the potential risks, consequences and benefits of building one based on known scientific facts.
When discussing future technology it is harder to know where to draw the line, but answers should still be rooted in what we know of the universe and be compatible with the current state of scientific knowledge.
Most answers on Worldbuilding are expected to be based in logic and, to some degree, science by default, so even questions without this tag may receive scientific answers. However, use of this tag indicates that the asker wants specifically science based answers; answers based in magic or pseudo-science on this question are not answers and may be deleted.