Hello,
The bolded lines will be very the darkest black lines on the design while the "solid" backstitch lines will not be as bold. You can always refer to the picture on the cover for reference but these bolded lines are the ones done in yellow (742). So look at the "TACO" wording and the the circles that are yellow in color and those are the "bolded" backstitch lines. You will see that they are much darker and thicker than the other solid backstitch lines on the chart. Hope this information helps :)
~Kristi @ ECS