What is the Number field?
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Part of our inbuilt issue relation tracking, associated with the type field next to it. Totally not clear, I know.

I would start by moving the Save and Cancel buttons from the subnav to the breadcrumbs
Just as we moved the edit button
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What is the Number field?

It starts to make sense when you select a "Type" like

Totally not clear, I know.
Not sure if we're going with this approach, use the GitHub "references" or even labels ?

Moving the Save and Cancel button sounds good - I think we need some "common area" where to place those buttons. Breadcrumb just doesn't feel right... but I think that's the choice for now.

Thinking about it now and how the two fields are connected to each other and how best to express that within the ui

How about changing the field labels to this

And increasing the "gap" between the previous fields to visually indicate that these two fields are connected and separate to the others

Sounds good. My only concern is that the tracker might be a "Feature tracker" (can a feature be related to another ? ...) so maybe we should put "Item" instead of "Issue" ?
As always - not sure about the wording...

Off Topic
I'd like to quote @eddieajau from another thread here (and this is not - in any way - meant as an offense !)

@brianteeman: Please clone yourself, we need more of you.

Yes a feature can be related as the PR would be related to the issue
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What if a ticket is related to several issues?

What if a ticket is related to several issues?

That can be considered a limitation but it also depends on what the relation actually is. I mean a "duplicate" can not be a duplicate of another.
The feature is also meant as an aid, so that you can focus on one issue (where the "related children" will be displayed)

I tried to explain the limited possibilities in #76 and I think there is a lot of space for improvements ;)

Moving the Save and Cancel button sounds good - I think we need some "common area" where to place those buttons. Breadcrumb just doesn't feel right... but I think that's the choice for now.

For me it's very suitable place.