Should and Could. Both are used to indicate the possibility of a situation then what is the difference?
"should" often implies obligation, just the same as "must", but tones it down. In regular conversations, "should" is common, unless spoken with authority.
"could" implies possibility or a soft recommendation.
You should (must) apply for the position.
You could (you possibly can, or have what it takes to) apply for the position.
Although commonly heard "could you help me?", it invites the pedant's response, "I could but I won't." Speaking and writing clearly are difficult. It is a good rule to say what one means. A bank robber doesn't ask, "could you put the money in the bag?" All this aside, I recognize "could" is used to make a request more gentle. But that can be accomplished without murdering the language, "would you be kind enough to ......?"