The front facing one pulls the cabinet against the wall when screwed in.
The bottom one makes up/down adjustments.
Screw the front one in until the cabinet is flush to the wall, then release it half a turn so the cabinet can slide up and down.
Then put a spirit level on top of the cabinet and adjust the bottom screws on each side.
After every adjustment bang the top of the cabinet so you know it's sitting fully on the brackets.
Once you're happy with horizontal level, screw front screws fully and check vertical level.
Hopefully your wall is levelled, otherwise you'll need to adjust front screws.
In some cases, you will need spacers on bottom rear corners of the cabinet if the wall is leaning forward.
It's ok for vertical level to be slightly out of not going in a corner, but you need to check when all cabinets in that line are fitted and find a compromise.
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