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1:
solve(u *x < a,x) returns
Warning! Solving parametric inequation requires assumption on parameters otherwise solutions may be missed...
[]

2: assume(u<0 and a<0) returns
a // shows the last assigned value

3:
solve(u *x < a,x) returns
Warning! ...requires assumption ... // The warning has to be dissolved, if it finds at least one assumption or better warnings about the variables not found ... The variables 'a,u' has no assumption
list[x>(a/u)]

4:
purge(a);
assume(u>0)

5:
solve(u *x < a,x) returns
Warning! The variable 'a' has no assumption
list[x<(a/u)]

You can not assume with and on two parameters, you must do 2 assume commands, one for each variable.
The warning is a generic warning, it means that solving parametric inequations is dangerous and not well supported, the user should verify the result.