MB229.51 What Does It Mean?

Mercedes test a bunch of different oils from various manufacturers to make sure that it'll be OK in their DPF cars.

They check for correct advertised viscosity, base oil type (synthetic, semi synthetic or mineral),
ACEA rating and API rating (Standard European and American approval),
SAPS ratings (Sulphated Ash, Phosphorous, Sulphur).

It's the low sulphated ash content that is very important about MB229.51

They ensure that they can lubricate properly using a few pieces of test equipment like a Falex tester.

Why Do They Have This Standard?

This standard was put in place so that the Mercedes garages all over the world were given a set choice.
Given that most of the Mercedes garages in the world are franchises, the majority (if given the chance)
would increase profit by using the cheapest oil they could possibly get. The cheapest oil would cause
diesel particulate filter failure very quickly if any oil was being burned in the combustion cycle.

This way, Mercedes could monitor the situation and ensure that warranty work isn't being
done because garages were using crap oil which was breaking the customer's car.

Do I Have To Choose From This List?

Currently I'd fully recommend you choose from this list if you are doing an oil change on a 54bhp diesel 451.
If you use MB229.5 spec oil, it won't have the low sulphated ash content and that will screw your DPF in no time.

Can I Use MB229.51 In My Petrol Car?

You can but it's not recommended for 2 reasons.

Firstly, MB229.51 is more expensive than the MB229.5 oil that you should be using.

Secondly, The MB 229.5 (higher SAPS) oils are actually longer lasting
due to the higher initial TBN (acid neutralizing ability) of the engine oil. 

So, unless you have a diesel with a DPF, don't use MB229.51

MB Specification 229.51

This is an addendum to the MB229.5 oil specs for all other smarts. 229.51 covers the
oils necessary for DPF equiped cars. Here is the official list given out by Mercedes.

There are plenty of Mercedes vehicles that have a DPF however, smart only has the 54bhp diesel model.