Modelling complex objects consisting of many separate parts, for each of which the Subdivision Surface modifier is added, it often needs to increase or decrease the number of modifier subdivisions at once for all of them. This can be done by selecting them and changing the modifier parameter value with the “alt” key pressed.
Sometimes it is necessary to add a single modifier to each part of some complex object. The easiest way is to cycle part by part through all the object and add the desired modifier to each its part manually. But if the object consists of many parts, it will take a lot of time.
There are some ways to greatly facilitate this work: