mscx confusion with ties on element <notes>

I recently had a look at the mscx files of my scores. Inside these files when there is a Tie() there is sometimes an element with a value 1 or -1.
When I remove this element it causes Problems but I dont understand what it is there for.
Can someone maybe help with this?

Here's a full example of a spanner like this:
<Spanner type="Tie">
<Tie>
<eid>4372276707359</eid>
</Tie>
<next>
<location>
<fractions>1/64</fractions>
<notes>1</notes>
</location>
</next>
</Spanner>

I know the mscx file isnt documented but I wanted to try to understand it anyways and this got me really frustrated for a while so thanks for any help <3