The sum of the decibel is based on this formula:
$$ L_p = 10log_{10} (\sum{10^{\frac{(L_pi)}{10}}}) $$
The substract of the decibel is based on the following formula :
$$ L_p = 10log_{10} (10^{\frac{(L_pi)}{10}}-(\sum{10^{\frac{(L_pi)}{10}}})) $$
The average (avg) of the decibel is based on the following formula (for example average on a surface - ISO 3744) :
$$ L_p = 10log_{10} (\frac{1}{N}\sum{10^{\frac{(L_pi)}{10}}}) $$
Remarks : It's not possible to mix the different operations.