One of the most difficult aspects of Mobile Frame Zero to grasp is tactical and combat order. I'm going to go in depth on this set of rules to try and explain them.
Let's start with Tactical Order.
Per Pg 90 of the rulebook, "The round begins in tactical order. In tactical order, always start with the player with the highest score." At this point there are no frames activated and no pending "combat chains" so the action goes to the player with the highest score, the defender.
The defender has two options, activate a frame, or pass. If the defender passes it then goes to the player with the next highest score. If they pass it goes down one more, and so on until it reaches the player with the lowest score. They cannot pass and must activate.
Once a frame has activated, it declares it's target and rolls it's dice. After assigning dice and making an attack, the defending frame declares it's target and rolls and assigns it's dice, unless the frame has already acted this round. The attacking frame then resolves it's turn and then the defending frame resolves it's turn. If the defending frame attacks a third target, that frame may also activate, causing a combat chain which is what is considered Combat Order. This is when frames act out of Tactical Order due to attacks made against them. Once the combat chain is resolved and there are no more frames who need to finish their turns, things go back to Tactical Order, going back to the player with the highest score (which may have changed if a frame was destroyed or a station was captured).
Lets go through an example.
The defender has the highest score and activates Frame A in Tactical Order, and declares Frame B as the target in direct fire range. After rolling it's dice and assigning them, Frame A attacks Frame B. Frame B then declares an attack on Frame C, then rolls and assigns it's dice. Frame A finishes it's turn, dealing damage to Frame B if applicable. Then Frame B acts in Combat Order and attacks Frame C. Frame C declares an attack against Frame B and rolls and assigns dice. Frame B finishes it's turn and deals damage if applicable, and then Frame C acts in Combat Order, attacking Frame B. Since Frame B has already acted, after Frame C finishes it's turn Combat Order ends. After Combat Order ends, Tactical Order begins again, allowing the player with the highest score to pass or activate.
If Combat Order never begins (for example the frame attacked has already acted), immediately after the frame finishes it's turn you return to Tactical Order. This means that the defender could, in theory, activate every one of their frames without anyone else at the table getting a chance to activate theirs.
Hopefully this helps clear things up for you. If you still have any other questions, feel free to ask!