Question:

What is the difference between a tanks, parts and a components?

Answer:

A tank is a Part (but not all parts are tanks -- the hull is also a
Part).

Each Part must consist of at least one Component but it may consist of
more than one Component.  Regardless of how many components are in a tank
it behaves as one unified tank when calculations are done with it.

Components may be adding or deducting.

Therefore, you can make a tank out of more than one component:  some
components can add to its volume and some can subtract from its volume.

The best way to make an engine room, for example, is to create the
overall space and then deduct various tanks from it.  For example,

  CREATE ENGRM.C
   PERMEABILITY 0.85
   ENDS -50, -70
   FIT HULL
   DEDUCT DOTK3.S
   ENDS -55, -70 
   TOP 4
   INB 1
   FIT HULL
   DEDUCT DOTK3.P
   ENDS -55, -70
   TOP 4
   INB 1
   FIT HULL
  /  

If you want to create a tank which combines two spaces, you can do it
as in the following example:

  CREATE LBT4.S
   PERMEABILITY 0.95
   ENDS -90, -110
   BOTTOM 2
   FIT HULL
   COMPONENT LBT4A.S
   ENDS -85, -90
   BOTTOM 2
   FIT HULL
  / 

There is seldom any need to JOIN components.  If you do use the JOIN
remember that only Components within the same Part can be joined to
form one Component.