How to apply models when making maps
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1e. Models and scanfiles
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Models can exist in two places:

   1e. As a list of model parameters in a Model (MDL) file
   2e. As a list of model parameters and complex uv-data points
         in a Scan (SCN) file

The model-list and the uv-model in a SCN file are always in accordance 
with each other. 

Models are update in the SCN files after a respons to the MODEL_ACTION 
prompt. MODEL_ACTION is in fact: "What model is actually going to be used
for processing and what model will finally end up in the SCN file?"

The MODEL_ACTION question is asked in many cases:
  1e. After the NMODEL option SAVE has been choosen
  2e. After the NMAP option SUBTRACT has been choosen
  3e. After the NCALIB option REDUNDANCY has been choosen
  4e. After the NFLAG flagging operation ARESID has been choosen

In all cases, the MODEL_ACTION is preceded by the MODEL_OPTION prompt.
This allows you to create a temporary model list in memory by reading
MDL files or by creating model components "by hand". The temporary model 
list can be editied as well.

The MODEL_ACTION determines what model is actually used in the 
following processing AND what model is stored in the SCN file

                         Used            Stored
MERGE                  SCN ^ List       SCN ^ List
ADD                    SCN + List       SCN + List
NEW                      List             List
TEMP                     List             SCN
INCR                   SCN + List         SCN

SCN:   Model currently present in the SCN file
List:  Model currently in memory (constructed at MODEL_OPTION)
 ^     Combination of two models, with common components occuring only
         once in the final model
 +     Addition of the two models, components occuring in both models
         are added as well, so they get the double amplitude

Obviously, options MERGE and NEW will be most commonly used.

Apart from this choice, MODEL_ACTION lets you decide how the model should
be calculated: should band smearing and time smearing be taken into account,
should instrumental polarisation be taken into account.


2. Subtracting models with NMAP
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There are two ways to subtract a model from uv-data before a map is made:

  1e. In NMAP, answer SUBTRACT=YES

      You will be asked for MODEL_OPTION to construct a temporary list,
      and for MODEL_ACTION to determine what model is to be used.

      This way you cannot use just the model that is present in the SCN file.

      Also, you cannot use different models for different sets or different
      SCN files (NMAP allows you to combine various SCN files, and to make 
      several maps based on several groups etc)


  2e. With NMAP, use DEAPPLY=MOD and SUBTRACT=NO

      The uv-model in the SCN file(s) will be subtracted from the uv-data 
      it corresponds to. The uv-model is considered a modification of the
      visibilities corresponding to an empty sky. Deapplying this 
      modification thus corresponds to subtraction of the uv-model.

      This way, you can only use the model that is present in the SCN file.
      This way, you can use different models for different groups in a SCN 
      file, and different models for different SCN files.

      However, you cannot add sources from another model, and you cannot
      control the BAND/TIME/INPOL parameters (you should have done this when
      you put the model in the SCN file).

The same reasoning applies to NCALIB and NFLAG.