Check MB 11-04 to see if it's a virtual extension. Also check MB 47-03 to see if it's a group mailbox.

I looked it up, but its neither a virtual extension nor a group mailbox

Search through MB 11-1, 11-2. Try to find 513 and see what feature it's assigned to. This may help figure this out.

No,neither in MB 11-1 nor 11-2. I do however see 513 as a received number further down in the ddi traslation table as a separate entry. It reverse to transfer target 2 --> 400 ...

I don't know why soembody would have programmed it that way, but if you are getting the number as a received number later on, you are probably using virtual loopbacks. See if you have anything cofigured in 10-42 ,and if in 11-07 you have 51 configured as a department group pilot.
I have no clue why the system would be configured as such, because I can't think why the original DDI translation just goes to 400. If it is set up as a virtual loopback, it should be breaking the customers caller id of any call coming to that number, because a virtual loopback would make it all appear as "513".

MB 10-42 is indeed configured with:

01 - Loopback Port Count 30
02 - Logical Trunk Port 31
03 - Logical Telephone Port 41
04 - Layer 3 Timer Type 1
05 - Calling Party Number Announcement
06 - S-point DDI Digits 0
07 - S-point Call Busy Mode Alerting Message

In MB 11-07 I see department group 13 referring to pilot 513.

I am not familiar with this. Can explain how the call flows and why? Also, what does the 400 mean in de ddi-table?
Many thanks.

Huh that virtual loopback is setup dumb..... 0 additional digits.
How virtual loopbacks work is calling an extension calls the associated trunk. The caller id given then presents itself again to the DDI table. This way it is possible to do things that can only be done from the a trunk call internally. Though normally you are supposed to add an S-POINT DDI digit, which is additional digits that you can type after calling the department group to have the virtual loopback be able to do more than one thing.
https://www.youtube.com/watch?v=yzX9pS-jojM This video from NEC explains how to set it up on a SL2100. Please note though that if you phone SL21 support they will tell you that it's not supported, and to unprogram it before they will help you with anything else. Something about idiots ruining their caller id. No clue for the SV81 though, I presume if the feature has survived to the SL21, then it probably was "supported back then".
I would try configuring the DDI translation for the original DDI the same as the translation for 513 and see if it doesn't break anything. I have no clue about transfer target 400. The system says that's VSR, which I have no clue what that is.

Thanks for the explanation and the video Fodrod. I will have to study this subject to get a better understanding of it. Perhaps Belevedere can shine his light on tranfer target 400.

By the way is there a training course available to get a general overall understanding about the system. I do have the programming en feature en specs. manuals, but those are to detailed.

NEC has official training courses that cost like $60 available in NEC NTAC (Which you need NEC Anytime first). Though they really don't train much other than some random basics, and not all the basics. They really just give access to support.
I've found that 4comms VOIP on youtube has some decent videos on NEC and panasonic, but he has videos on random topics that he's made a video on, no comprehensive series.

Thanks for your suggentions Fodrod. I hoped that someone would have a trainingscourse availabe in powerpoint or pdf.

You can find quite a bit on you tube.

Thanks for your suggestion Belevedere.
Do you have any idea about how target 400 end up on a vrs message?