That's a historical artefact. In the Read vocabulary, [X] meant concept derived from ICD-10. When SNOMED-RT and Read were merged, these were all poured into the new SNOMED-CT, and then later sequestered into the SNOMED UK-Version (which we implemented because of the Read codes in CPRD and THIN). SNOMED is phasing them out. Sooner or later they will be gone entirely.
Same is true for [V] and [Q] (observations and qualifiers from ICD-9), [SO] (anatomical sites from OPCS) and [M] (morphology terms from ICD-O).
Don't use them, just ignore them.