The NZXT Source 530 has three sets of removable drive cages (which can fit either a 3.5" HDD or a 2.5" SSD), stacked on top of each other, located at the front part of the case. The number of HDDs/SSDs that can be fitted in each of the three removable drive cages are (from top-to-bottom): [3x HDDs/SSDs] + [2x HDDs/SSDs] + [1x HDD/SSD]
Notice that the topmost cage (the one that fits 3x HDDs/SSDs) has a fan mounting slot ("pivot fan") for optional additional airflow, while the rest does not.
In the NZXT webpage (https://www.nzxt.com/products/source-530), it clearly states that:
"
Clearance (VGA Card): 282mm (With Cage & Pivot Fan), 310mm (With Cage), 444mm (Without Cage)"
This means that you can still fit a 300mm-long GTX 1080Ti even with all the cage installed, as the case can fit up to 310mm (without the Pivot Fan installed).
As these drive cages are removable, you can also select which specific drive cages you want to remove and retain. For example, if your GTX 1080 Ti would be more than 310mm, you can simply remove the top cage (the one that fits 3x HDDs/SSDs) and still have the bottom 2 cages in there (the one that fits 2x HDDs/SSDs and the one that fits 1x HDD/SSD).
Note also that behind the motherboard tray, there is an additional dedicated slot for a 2.5" SSD.