The preceding post got it right. Top left is the step count, top right is the battery status, bottom left is the heart rate and bottom right is the UV index. These are the default complications.
The customisation is also correct, but if you want to stay with the rest of the design, you can't just chose every feature you want because not all are dispayed as a bar. Some will be displayed as a text that goes around the corner and look bad, if I recall correctly.
