What's is the difference between relative abundance vs percent abundance

Relative means you take the required amount from the actual amount which gives you the excess or abundance, its relative because you compare it to the required amount.
The percent abundance is the relative abundance as a percentage of the required amount.
Therefore % abundance= (abundance/required amount) X 100

The key difference is that percentages are standardised as a number out of 100 which allows you to compare them with other percentages without any calculations.

Hope that helps.