"Shared sheath" means that you can use one multipair copper cable for different applications. For example, pair 1 transmits voice, pair 2&3 transmits data, pair 4 transmits video.
If a cable supports "shared sheath", it means that the manufacturer has tested different applications sharing one cable and issued a list of compatible applications for the particular cable type.
In general, applications using totally different frequency ranges tend not to interfere with one another within the common sheath (good example is voice and 10Base-T).
Roman Kitaev, RCDD