[nanskris]

Q1

Identified future growth area
Absolute skills shortage
Long Term Skills Shortage

How are they related to each other?

Q2

I have a BA degree and an M.Sc degree in Computer Science. I am an IT professional. Naturally, my BA is not a related qualification. Can I claim points based on my MSc [Degree 9] instead of a degree level [Level 7] for Long Term skills shortage requirements as mentioned in the Long Term Skill Shortage List?

Q3

Does my work experience necessarily have to be post qualification? I have been working as an IT professional even before I signed up M.Sc.

[BkyMonster]

Lets see if I can answer some of this.
Q1.
Area of future growth will get you a few points. There are 3 areas of future growth. If your experience/qualification falls into one of them you can claim those points.
Absolute skill shortage list and Long term skill shortage list are the same thing ( I think?), but not the same as the Immediate skill shortage list.
The first is based on nationwide shortages and is something you can claim points for on your EOI. The second is more regional and can be of help when you are trying to get a general work or talent visa. No points can be claimed on the EOI for Immediate skill shortage list. IT is on the former so you should not worry.

Q2. You should be able to claim points based on your MSc. The points you get from your MSc are dependent on your level 7 qualification being assessed as a level 7. I don't believe it needs to be a related qualification, just the correct level.
As soon as your EOI is selected I would recommend looking into whether or not you need NZQA assessment of your degrees. There is a section on this forum about it. It takes a while so start early.

Q3. Work experience does not need to be post qualification, though it should be full time in a comparable labour market.

I hope that helps somewhat.
Good luck!