The standard catalogue is split into two volumes, which together covers over 1100 pages! This is illustrated throughout and cover every variant, some locos have 20 or more variants! Volume one covers locomotives, volume two covers coaching stock and goods wagons.
Then there is the "Kompakt" edition which is pocket sized, but has no illustrations, often used at model railway fairs and I mark mine off as a checklist.
A 4th catalogue, which I dont have, covers special editions of models.
Roughly speaking, catalogue numbers are as follows, three digits cover the early period 1935 until the late 40s, four digits cover the 50s until about 1975 and five digits cover the period from the late 70s onwards. This is not always correct, but I use it as a rough guide when looking at Märklin products on the second hand stalls.
Since digital has come in, analogue locos have taken a plunge in collector value. In the last 20 years they have halved in value, the exception being very rare items.