# Collection types
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Collection types provide a way to organize and structure data for efficient retrieval and analysis. They are used in ML databases to define the schema of a dataset and its elements. In Amazon SageMaker Feature Store, the supported collection types include list, set, and vector. 

Collections are a grouping of elements in which each element within the collection must have the same feature type (`String`, `Integral`, or `Fractional`). For example, a collection can contain elements with all of the element feature types as `Fractional`, but a collection cannot contain elements with some feature types as `Fractional` and some feature types as `String`.

Only `InMemory` online store feature groups currently support collection types. The following list describes the collection type options.

**List**: An ordered collection of elements.
+ The length of the list is determined by how many elements are in the collection.
+ Example: You can have a list such as [‘a’, ‘b’, ‘a’], because the list preserves the order and can have repeat elements.

**Set**: An unordered collection of unique elements. 
+ The length of the set is determined by how many unique elements are in the collection.
+ Example: You cannot have a set such as [‘a’, 'b', 'a'], because it contains a repeat element. The set will instead have the elements [‘a’, ‘b’], because the set only contains unique elements.

**Vector**: A specialized list that represents a fixed-size array of elements. The order of the elements hold significance, such that the positions of the elements represent certain properties of the data. 
+ The elements in the vector collection type *must* have the `Fractional` feature type. 
+ You may only have one vector collection type per online store `InMemory` tier feature group.
+ The dimension (number of elements in the vector) of the vector is predetermined by you and is specified using `VectorDimension`. The max dimension limit is 8192.
+ Example: You can have a vector such as [4.2, -6.3, 4.2], where the first, second, and third elements can represent the x, y, and z positions in physical space.

There are no limits on the length of the collections, as long as they don't exceed the maximum size of a record. For the maximum size of a record, see [Quotas, naming rules and data types](feature-store-quotas.md).