vpasolve with added variable 'u' in solution

I am solving a set of 9 equations and 9 unknowns. Using 'solve' with the symbolic toolbox, everything works perfectly. For a simplified case, I assume that one of those unknowns is known, and remove one equation that is relevant to that variable. Using 'solve' again with 8 equation and 8 unknowns, I get the error:

Warning: The solutions are parametrized by the symbols:

u = R_

v7 = R_

w5 = R_

x15 = R_

x23 = R_

> In solve at 190 In efficiency_pure_methane at 38 etaF = x23

I tried to solve this same set of equations with 'vpasolve' and got a solution. However, the solution for the unknown that I am interested in now has a variable 'u' that is NOT a variable I have declared. What is this 'u'? Does it mean that I have infinite solutions for any value of u like the 'solve' solution may suggest? The solution is 12 lines long but the beginning looks like:

(0.5*u*(4.0*M_R*M_f*M_inf*dX_CH4...

Any insight would be appreciated!

Cheers