First, let me start at the firewall. That detent is TOGA. It is FIXED thrust. It's all the engine can deliver. Period.
Second, if you pull the thrust levers back to the next detent, you will be in the FLEX/MCT detent. The way this detent works is as follows: If, before takeoff, an ASSUMED temperature is entered in the FLEX prompt of the PERF page, the thrust levers will deliver a reduced takeoff thrust that will be at least EQUAL, but never LESS than climb thrust. The fadec takes into account the OAT when it AUTOMATICALLY adusts EPR or N1 (depending on the mode of the IAE engine or if you happen to have the CFM engine) because it has an AIR DATA input through the ADR. Now, if you have performed a takeoff, and you move the thrust levers to any other detent and then move them back to the FLEX/MCT detent, the fadec will command, and you will get MCT and nothing else because it is a FIXED thrust. If you are using the MCT for some reason, and you select a faster or slower speed while climbing, the elevator will pitch the airplane to make good that speed. If you are in level flight, the airplane will keep accelerating and selecting a speed will have no effect. Thrust is FIXED. Period.
Third, the climb detent. The climb detent is where you put the thrust levers when you transition to the climb phase immediately after 1000 feet AGL or whatever altitude you decide your transition to be (i.e. 1500 AGL for noise abatement). Once the thrust levers are placed in this detent, the AUTOTHRUST will command thrust anywhere from IDLE, to MAX CLIMB thrust depending on what speed you select on the FCU if you are using selected navigation (i.e. using the knobs on the FCU to select HDG and SPEED and ALTITUDE). It will use the same range from IDLE to MAX CLIMB if using the FMGC to MANAGE the airplane in CLIMB, CRUISE, DESCENT, and APPROACH phase of flight. So, if you are in LEVEL FLIGHT with the autopilot engaged or are hand flying, and you are going 250 knots and you select 200 knots let's say, then the autothrust will command IDLE until you reach 200 knots and then it will increase thrust to maintain 200 knots. If you are going 200 knots and select 250 knots, the autothrust will command MAX CLIMB thrust until reaching 250 knots and then back off to the point that it needs to in order to maintain 250 knots. Now, if you are descending and you have 200 knots SELECTED, yet you are hand flying and the speed is FASTER than what is SELECTED, the autothrust is going to command IDLE. Under this circumstance, if one were to select MCT or TOGA, the thrust would immediately go to that value and would be an excellent indication of whether the engine was A) RUNNING and B) whether it would be capable of producing more than IDLE. Unfortunately, going to a fixed thrust detent in order to assess whether the engine is running or not isn't anywhere in the Engine Dual Failure QRH checklist.