A semantically partitioned object is an InfoProvider that consists of several InfoCubes or DataStore objects with the same structure. Semantic partitioning divides the InfoProvider into several small, equally sized units (partitions).
SAP often uses the picture below the explain the SPO principle.

SPO based model

SPO based model

As of BW 7.3, creating SPO based InfoProviders has become much easier due to the SPO wizard. Unfortunately this wizard is one of the best kept secrets as it hasn’t received it’s own transaction code….yet
For creating an SPO based DSO, the checkbox ‘semantically partitioned’ needs to be checked. (The same applies for SPO based InfoCubes)

Checkbox semantically partitioned

Checkbox semantically partitioned

After having checked ‘semantically partitioned’, the following ‘wizard screen’ appears which assists you in creating, in this case, an SPO based DSO

Maintain partitions

Maintain partitions

Click on maintain partitions to define the partition criteria. The first step is to choose the infoObject(s) on which the partition criteria is to be defined.

Select partitioning criteria

Select partitioning criteria

For more information regarding the SPO wizard I’d advise you to read to whitepaper about it. You can access this by clicking here