10 centimes German occupation

I have recently purchased 2 of the above mentioned coins 1915 and 1916.
My question is with the 1916 selections KM#81
What do the notes: "date letters "1916" and date letters "1916" overdate variety exist
mean.  Please explain so that I choose the correct selection.

It means that the date might be at the top of the coin rather than the bottom.

So am I correct in " Date letters 1916" is at the bottom and " Overdate" the 1916 will be at the top.

If it says date letters...that usually means that they are using Roman Numerals for the date.
Overdate variety exists means that you can also have the date at the top of the coin.
I believe this is correct.

I have not seen the coin or page you refer to, but I thought the term 'overdate' meant that the mint made a mistake (or reused a coin with an earlier date); so they stamped the correct date on top of another.
 Usually it is the last number of the date that is over-stamped (such as there is a UK penny from 1865 that was stamped over the date 1863 so is known as 1865/3). Below is a more unusual variety where the third number is amended - it did read 1922 and was overstamped with the correct date of 1942.

The coins I purchased look the same as the example KM#81 in the catalogue.
Which line should I select for the 1916 coin as it is not in roman numericals and has no overdate.

You mention German Occupation, but what country is your coin? Perhaps Belgium ...

https://en.numista.com/catalogue/pieces514.html

 If so then I see on that page there are varieties of 1916 with a dot in various positions, or not. So if yours has both dots then I would enter it in the 'overdate variety exists' part. Which does not mean that you have an overdate, it is just a note to anyone who has one. So that part is both for your coin and the overdate.
  Finally, as this was issued for only three years, I guess the overdate would be where a 1915 coin had 1916 stamped over it (so 1915/6), as I have not heard of a coin being back dated (so here 1917/6). I think the mint does not re-strike the whole coin, just the date part.

Yes, it is the Belgium.
Looking at the picture that was sent to Me by the seller, it has a dot on either side of the year.

I am collecting the coins tomorrow.  So the best will be that when I get home I will take pics and add it to this post so You can give a accurate selection for the coins.
Thanks for the responses

Yeah, it's a date stamped on top of another date. I've got a Belgium coin myself that has an overdate. A 1905/06 where the 5 is rounded into a 6. I think it's a feature you can find on a few early 20th century Belgian coins.

Ok, sorry to have confused you.

   I didn't know all that...so thank you.