Inputs
When connecting a distribution, this setting does not need to be used, because it consists of different data types that the node can recognize automatically. However, if you want to decode a container only for data that has a certain type, you can choose here among the common data types, such as Float, Integer, Matrix or Vector.
If you need even more control over the read data type, you can use the small arrow button on the far right to open a menu with more choices. In the "Basic" group you will find the data types that can also be called up directly via the large button behind Data type.
Under "Advanced" you find beside some additional data types also more concretely defined basic data types in 32 bit and 64 bit variants.
At "All Modules" the data types are sorted, which Cinema 4D itself uses. These are often special types that you will probably need less or not at all.
The same applies to the rather complex data types that can be found in the "Structure" group. But these are again interesting in the "Compose Container" node, which is the counterpart to the "Decompose Container" node. In this way you can create your own containers, e.g. from data arrays. See the description of the "Compose Container" for how this works.
Once a data type has been selected, it can be viewed in a separate dialog by selecting "Edit Data Type..." and edited there if necessary.
Container In
Here you connect the container whose values are to be read out. This can be, for example, the "Distribution" output of a "Distribution Op" node.
The outputs of the node then update automatically to display the arrays of the container. However, by selecting a specific Data Type, the container can also be searched specifically for data of a particular type. So with the data type "matrix" with a read distribution also only the outputs for off (position vector) and sqmat (matrix, which reflects only the size and orientation) will be made available.