What is "OST1" for Alt text in Storyline 360?

Hello,

I understand that OST stands for "On Site Text" but does that mean that if it's there as Alt txt, that by default the screen reader will read whatever the content for this specific text box which means I don't have to copy the content of the text box and replace the OST by the actual text? 

In the screenshot attached, I have "OST1" twice for 2 different text boxes. Why is there a number following the OST and if it's ok that both fields has the same OST1 or should I rename the 2nd one OST2?

Also when the Alt text is "Title" so the screen reader will read the actual title from the content without me having to include the actual title in the Alt text?

I would really appreciate your input on my questions above.

Thanks,

VF