You have most probably seen such notes in the parish books - cause of death and then a number of a death survey card.
(Senility according to the death survey card from June 3, 1873, no. 11.
These survey cards were issued for every cause of death since the first half of 19th century - exact date when they were mentioned in the parish books is different for each parish office, but we can see such notes in almost all of them after 1840.
What was written in these survey cards?
Some of those cards were left in the Southern Bohemian parish books and then scanned when the parish books were scanned. Here is one of such cards from 1862:
Death survey card
Name and surname: Josef Šimeček
Birth place: Doudleby no. 27
Residence: Doudleby no. 27
Religion: Roman catholic
Age: 2 years 3 months
Last illness: measles
Day and hour of death: July 5, 1862 at 10 AM
Day and hour of burial: July 7 of the same year at 8 AM
In Doudleby on July 6, 1862. Petr Birkl, councilman
As you can see there is quite a lot of information which is then written to the parish book. There was no medical doctor called for the child - as measles belonged to the most common causes of death of small children the doctor, which was quite expensive, was not usually called in such cases.
I have seen mentioned that the numbers in the cause of death column refer to some list of diseases and give the number of disease. This is not correct. The number in the cause of death column is a number of the survey card, nothing else. There were no official lists of diseases till the 1st half of 20th century.
The death survey cards are usually not available online - the card above is one of the cases when the card was forgotten in the parish book. These survey cards are usually stored in the parish archive in the State District Archives, so you would have to look for them offline.