Difference between unisgned(a,b) and to_unsigned(a,b)?

-- A standard-logic vector, where bits mean anything
signal slv : std_logic_vector(7 downto 0) := X"0F";
-- An unsigned integer, where bits mean positive numbers, eg., 0->255
signal u : unsigned(7 downto 0) := X"0F";
-- Conversion of std_logic_vector to unsigned
u <= unsigned(slv);
-- Conversion of unsigned to std_logic_vector
slv <= std_logic_vector(u);
-- Conversion of an integer to unsigned of width 8-bits
u <= to_unsigned(123, 8);
-- Conversion of an integer to std_logic_vector
slv <= std_logic_vector(to_unsigned(123, 8));

unsigned(a) - note: not (a,b) - is a type conversion of a similar type a (ie. another array of std_logic, like signed or std_logic_vector.

Could you please explain this line of code:

its just a constant with the value CTRL_OFF, with the width ADDRESS_WIDTH. It is an unsigned number

Thanks a lot tricky and dave