In legacy code, I encountered an unknown attribute spsourceindex, what is it used for? Example
<nav class="size-options">
<a href="#" class="any-class" spsourceindex="25">One</a>
<a href="#" class="any-class" spsourceindex="30">Two</a>
<a href="#" class="any-class" spsourceindex="31">Three</a>
<a href="#" class="any-class" spsourceindex="32">Four</a>
</nav>