Rules for firepower

When does these rules for firepower apply?
Giving orders is done by writing messages like "attack or assault this or that position, or hold, or advance ..." A player can't issue an order like "fire at this or that target" ? So, is it the umpire who decides to fire?
And when two enemy infantry lines are within shooting range, who's to fire first? Blue or red? (it's clear to me that, if one side decides to bayonet attack, the defender may decide to stand to receive the attack => they fire)

Umpire decides in the games I run and the players have to accept a level of abstraction where they have no control over the lower levels of detail....