Query on P60U and tax

So on my yearly P60U letter I get told the amount of benefits I have been paid for the year. I am in ESA IR Support Group, I presume this is old style as I haven't been transferred over to UC yet. I don't get any other benefits.

So it shows my benefit amount and then it says "£0.00" on all other boxes, so under "Taxable" it says £0.00. Same with the "Pay & Taxable benefit" box.

Does this mean it is not taxable at all, or just because it is below the £12K threshold ? If the latter, then why ? I thought ESA IR was non-taxable. If the former then why am I sent a P60U if it is not taxable ?

The tax codes mentioned on the letter is ESA500 and 1257L

I read that non-taxable benefits are not shown on a P60U so why would mine be ?? On my other yearly benefit breakdown form it says it is income related entitlement.

Are P60U's with ESA IR automatically sent to the HMRC ? It just says on the back "keep in case needed".

Thanks for any info.