FPAR is the general command to assign initial values and permitted limits to fit parameters.
Each parameter is addressed by a running number
(corresponding to the description of the respective fit function).
Alternatively, the commands FAREA, FPOS and FWIDTH are available for
peak functions, where areas, centres and full widths at half maximum are
specified by peak numbers.
Fit parameters are set to zero initially.
FPEAKS calculates guess values for parameters of peak functions
from sum and moments of corresponding data regions. 
The fit function may always be evaluated and displayed
with the actual set of parameter values (command FDISP).
 

Fixing fit parameters

 
A fit parameter is allowed to be held constant during iterations by
fixing its absolute value or the difference, sum, quotient or product with
respect to any reference parameter (which for its part must not be relatively fixed).
The guess value for a relatively fixed parameter may be
given in relative mode, i.e. by a difference, sum, quotient or product
which internally is transformed into an absolute value.
Fixing and varying is accomplished by command FPAR
or, for peak function parameters, by the commands FAREA, FPOS and FWIDTH.