It just means that strengths and weaknesses exist ona spectrum relative to one's own psyche....The numbers (+,-) are there to simply illustrate this relativity.
Kinda reminds me of the MBTI % score strengths of each function, a spectrum or think of it as shades of a particular color.
In addition,
When it comes to socionics dimensionality,
You can look at it as how easy it is for you to develop abilities in that area and maybe how confident you are with that area. Again, like a spectrum:
(Greater than) 4 D: “I know my shit.”
(Greater than) 3 D: “I am rather good at this.”
Greater than 2 D: ”I can handle this.”
Less than 2 D: “I think I need help.”
(Less than) 1D: “I don’t know I need (a lot of) help.”
So lets say you're trying to type someone, and you were looking at a questionnaire they filled out, you'd compare his functions amongst him/herself and not compared to a different person.
As for the 5th dimension ??? I have no idea