what isn't to understand? you meet someone, hang out a few times or have a few convos and assess them: do you like them? are they someone you can see yourself pursuing a serious relationship with? are they someone you're physically attracted to but not interested in being with for whatever reason? if this third is true, you then ask yourself, well could you have a physical relationship with them? if so, you may go the FWB route - single people need to get laid too. my FWB have been that only, i don't spend time on the phone with them, i'm not trying to bond or connect or become emotionally intimate, i'm not doing things for them nor expecting things from them. i want to link up, have sex, and leave/send them home.
if they are someone you're interested in the possibility of having a real relationship with - you put in effort to get to know them, plan dates, spend time, talk, open up emotionally, share, show affection, you show interest in supporting them or getting to know other aspects of their lives, meet friends, family, etc. i'm not seeing how you can't see the difference in the effort you put into someone you're seriously pursuing vs someone you just have a sexual relationship with.
i'm happily partnered