The PES block would have a CEmbRect object only. If a PES version 6 design, for example, contained a rectangle object. Later versions of PES have higher level objects which contain vector shapes and information about how these are filled when it renders. The PES data has a copy of the segments and the PEC data has a copy of the segments. So within PESv1 we end up with two copies of the same data in each of their respective blocks. These can be grouped in a few different ways but they only ever contain sewing segment information. These blocks contain the affine transformation data for the position data, and the position data. In PES version 1, the only blocks contained within the version 1 of the format were the CEmbOne and CSewSeg blocks.
However, later versions of PES version 4+ have addendum information about the PEC block that wouldn't interfere with the processing of the PEC block but contains information relevant to the embroidery machine.
Since the PEC block is the same in all versions so that it's always backwards compatible with the hardware of the Brother and Bernina embroidery machines, software made by others therefore tends to only read that data and turns those commands into line segments. And the PEC part contain the design name, colors, embroidery machine commands, and graphics for the embroidery machine. The PES part contains the design information for Brother PE-Design and perhaps similar applications.
PES files contain at their core two different parts for two completely different purposes. The PES format is a hybrid embroidery design and embroidery command file format for Brother Industries and Bernina International series of embroidery machines, among others.