Hi @Anonymous ,
I did a test on my side for you reference.
You can see below screenshot, I have added 5 checkboxes inside Siting DataCard :

Here is the code in Update property of Siting DataCard:
ForAll(
Split(Concatenate(
If(Checkbox1.Value, Checkbox1.Text & ",", ""),
If(Checkbox2.Value, Checkbox2.Text & ",", ""),
If(Checkbox3.Value, Checkbox3.Text & ",", ""),
If(Checkbox4.Value, Checkbox4.Text & ",", ""),
If(Checkbox5.Value, Checkbox5.Text & ",", "")
),
","
),
{Value: Result}
)

You can check result now, try to add a new list item through your app.
Besides, based on your own requirement, maybe you need to set Defaults property of each checkbox, so that when you edit an existing record, the corresponding checkbox will have the value.
So set the Default property of the SitingCheckbox to following(my control name is Checkbox1😞
If(Checkbox1.Text in ThisItem.Siting.Value, true, false)

Set the Default property of the SetoutCheckbox to following(my control name is Checkbox2😞
If(Checkbox2.Text in ThisItem.Siting.Value, true, false)

Repeat above step for all other checkboxes.
Best regards,
Allen