Let's take it one by one, what do you not understand ?
AC_START_0, _1, etc - represent some objects (in Blender I use empties) but really they can be anything (cubes, boxes if you wish). Just name them correctly, and assign them a material (any material from the scene will do). By naming them, SDK Editor will know they are just the spawn point so it will set them automatically non-renderable.
Here are 2 example from my scene 1)empty ; 2)boxes
Notice the axis orientation (just a 90 rotation on the X axis without applying transformations), it's important in order to avoid later spawn problems (broken wheels, pit guy under the floor, etc)
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AC_TIME_0_L (standing for left side, in the racing direction) and AC_TIME_0_R (for right side)
Just place them as some markers on the sides of the track, where you wish to have your split times. They will detect cars passing by (they will cover the entire space between Left and Right).
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For the PITS to act like pits, you need to separate that mesh, and name it differently (example PITS-ROUEN or ZONE-ENFANT or whatever).
Just remember to define its properties in the surfaces.ini file. It's important to mark it as pits zone:
[SURFACE_2]
KEY=PITS-ROUEN
FRICTION=0.95
DAMPING=0
WAV=
WAV_PITCH=0
FF_EFFECT=NULL
DIRT_ADDITIVE=0
BLACK_FLAG_TIME=0
IS_VALID_TRACK=1
SIN_HEIGHT=0
SIN_LENGTH=0
IS_PITLANE=1
VIBRATION_GAIN=0
VIBRATION_LENGTH=0
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For zones that aren't visible, don't worry; normally game will handle automatically.
You can however optimize the trees and vegetation, by using naming groups. More info
HERE.
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Voila, bon courage
