I've doing some research on the EIS and came up with this tid bit on an aftermarket equipment site.

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Equipment

Functions: 1. Reading of number, ID and status of a SMART Key for Mercedes Benz.
2.Testing of function ability of a SMART Key – both of Radio Frequency and Infra Red functions.

##### !!!!!!
3. Checking of a limit of starts of a SMART Key.
###### !!!!!

4. Radio Frequency volume detection.

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Then I find a further explanation ( rolling code that never repeats apparently )

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As it is known, hash is mathematical 8 bytes or a word, a coded word. And the level of
coding depends on the number of hash functions calculations.
The hash functions feature is one side coding. Let us have an 8 byte word like initial hash or password.

###### Using this initial (first) hash, we can calculate second hash, then third hash, using second hash etc. up to 200 000 hashes. ####

But hash function has one important property. We are able to calculate it forward only, hash has not back calculation. Then we can not calculate first hash when we have second one ets. CPU inside the key has 200 000 hashes and send them hash by hash from 200 000-th hash to 1st hash to the EIS.

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So, It looks like the key has a limited life and requires uninterrupted key send / EIS receive. I see a scenario where one data miss read cause the key to advance to the next code while the EIS is still on the old code and the car not operate.

From what I gather, other remote rolling code systems, like a garage door opener, IR / RF remote locks, use a rolling code block. There exists a list of codes in the computer and remote. The remote sends a code, the computer looks at the next one on the list but will accept a code XX down the list. This is done to account for remote button presses when away from the car.

The IR remote on my 97 SL320 has a sync feature where you cycle the IR remote at the car then insert key in ignition and turn on to resysc. Apparently the Smart key does not have such a feature.