Yes, t(0,15,0) means translate up 15. (move in y direction 15 meters)
Here's an easier way to code it, to understand better:
getDimensions--> # getDimensions rule receives a shape.
splitParcel # it sends a copy of that shape to split parcel.
t(0,15,0) # it moves the current shape up 15 in y direction.
Topside # it sends a copy of that moved shape to Topside.
# the rule is over and the current shape is no longer processed.
That shape left at the end of the rule remains in the "shape tree", but is not shown in the final model.
You can see the shape tree in the Model Hierarchy window (THE tool for debugging CGA).
For the official explanation of this, look in the CE help documentation, in the section pointed to in this image:
In fact, reading everything in the Rule-based Modeling section will help understand CGA a lot.
Chris