The fraction being constrained between 0 and 1 is a decision you should make. If you have 2 drugs you comapre the bioavailability, then there is no constraint for the fraction as we are talking about relative bioavailability.
Now if you have oral and IV data and you want to estimate the fraction going into the central compartment from the extravascular space, then you should constraint the fraction between 0 and 1.
It is easy with Phoenix.
The code is
you first define flogit as a parameter that is normally distributed . You need to go to graphical to do that.
1: define flogit as a parameter
2: define it as normally distributed
3: define f as an expression with f=ilogit(flogit)
4: click on the Aa compartment and look in the window you can edit and write f on the right of bioavailability
Put as initial estimate for tvflogit 0 which means 0.5 for f.
Hope it helps. The same argument can be used for any compartment you know the fraction cannot be >1 and less than 0.