I think the “Beibehaltungsgenehmigung” is approved for.the specific circumstances you are applying for, they are basically assuming you are going to apply immediately after receiving the document, which is why they give you 2 years, The document is actually specific to the nationality you are applying for and not any other. I applied for naturalization the day I picked up the document and was naturalized about 90 days later.
The part I found most annoying is that they keep no record of it, so every time I have to renew my German passport I have to show them the “Beibehaltungsgenehmigung”, at least that’s what they told me at the consulate.
