James wrote

Could you tell me what the unit is for the flux boundary conditions? e.g. for a 1D diffusion-advection model, the third type flux boundary condition can be -n*(-D(dC/dx)+uC)=No, which can be found in the boundary condition options. What is the unit for n here and what is the unit for No? I cannot find the description for n in the online materials. Thanks.

For boundary conditions "n := {nx, ny, nz}" are the components of the outward pointing normal vector, so unitless. The other units depend on your chosen unit system. FEATool in itself is designed without adherence to any specific unit system, so the unit of "No" and all other coefficients (geometry length scale, D, C, etc) all depend on what you decide them to represent. In SI units for example:

unit length scale ~ [m], C ~ [mol/m^3], D ~ [m^2/s], u ~ [m/s], d/dx ~ [1/m] => No ~ [mol/m^2/s]