A short trip is defined as any journey up to the fourth stop after boarding - if no more than two of these are made by S-Bahn or U-Bahn, ExpressBus or other included rail routes (e.g. regional train). You may combine different means of transport and change trains if you are travelling a short distance. It does not matter if you cross a zone border. Interruptions to the journey are also permitted - within the maximum journey time of one hour.
In the communities outside Munich, every bus trip within the community boundaries is considered a short trip. Regardless of the number of stops passed.
For a short-haul journey, you validate one strip of the strip ticket. Or you can buy a short-haul single ticket, the easiest way to do this is as a HandyTicket.
From Ostbahnhof to Maxmonument it is two stops with the S-Bahn to Isartor and two stops with tram line 16 to Maxmonument. So it's a journey of four stops, only two of which are on the S-Bahn - the short-haul fare applies.
By the way: It is not permitted to combine several short-haul journeys or zone and short-haul journeys with each other, but you can extend the area of validity of your season ticket by purchasing a short-haul ticket.
Please also note that the U21 strip ticket cannot be used for the short-haul fare. The short-haul fare can only be used with the regular strip ticket. The customer group of under 21s and children can also use the short-haul fare with it.