Those controls were declared already in the other partial class pages. You cannot access them in a declaritive context, but you can in a functional one.
public partial class frmPlanning
{
public void Test()
{
cboProjectCodes.Items.Clear();
}
}
Remember that all pages of the partial class are just continuations of each other, so you cannot just use variables/contraols that have been declared, but rather must put them in functions. You can also declare new variables that do not exist on other pages. Same with methods, you can call them from inside another method or overload the existing.