This is answered at the beginning of the chapter in which they appear.

In this section we describe each operator, function, set, and command that you are likely to encounter in CVX. In some cases, limitations of the underlying solver place certain restrictions or caveats on their use:

• Functions marked with a dagger (†) are not supported natively by the solvers that CVX uses. They are handled using a successive approximation method which makes multiple calls to the underlying solver, achieving the same final precision. If you use one of these functions, you will be warned that successive approximation will be used. This technique is discussed further in The successive approximation method. As this section discusses, this is an experimental approach that works well in many cases, but cannot be guaranteed.

• Functions involving powers (e.g., x^p) and p-norms (e.g., norm(x,p)) are marked with a double dagger (‡). CVX represents these functions exactly when p is a rational number. For irrational values of p, a nearby rational is selected instead. See Power functions and p-norms for details on how both cases are handled.

I think you would benefit from (re-)reading the whole CVX User’s Guide. It’s not very long.

yeah,you are right,it is necessary for me to read CVX User’s Guide,thanks very much .