Every unique value of data-fslightbox will be treated as instance.
<a data-fslightbox="gallery" href="...">
<img src="...">
</a>
<a data-fslightbox="gallery" href="...">
<img src="...">
</a>
<a data-fslightbox="gallery" href="...">
<img src="...">
</a>
<a data-fslightbox="gallery" href="...">
<img src="...">
</a>