It is a common operator. To find all available you can open the OCL-Editor and type in a class:

Your model is central to all expression you will handle. We will use this model to for the examples:

Operators	Description
Thing.allinstances	Gives you a list of all Things
Things.allinstances->select(someInt>3)	Only things with someInt bigger than 3
Thing.allinstances->select( (someInt>3) and (someInt<6))	Only things with someInt bigger than 3 but less than 6. Notice the extra parenthesis to or the Boolean expressions together
Things.allinstances->select(x|x.someInt>3)	Here we introduce the loop variable x. We separate the definition of x from the usage of x with the pipe sign “|”. Loop variables are optional but if names are unique – but you will need to use them to give precision or to if you want to perform operations on the loop context itself.
Things.allinstances.Details	Gives a list of all detail objects that are connected to a Thing. The Detail objects that float around without a Thing will not be in the list
Things.allinstances.Details.Attribute1	A list of nullable strings from the contents from the details attribute1. Note that OCL is null-tolerant – you do not need to check if the Details exists of not – the language handles null checks for you
SubClassThing1.allinstances.Details	Inherited features of classes are directly accessible
Thing.allInstances- >select(x|x.safeCast(SubClassThing1). OnlyAvailableInSubClass='Hello')	Filtering on Specialization is done with an operator SafeCast. This is null safe so for all objects that do not fit the profile the expression returns false