Which letter is not used as a sheet number prefix for certain plans?

In plan reading and development for the Florida Department of Transportation (FDOT), certain prefixes are designated for various types of engineering sheets. The prefixes indicate the specific category of the plan sheet, such as 'I' for index sheets, 'S' for structural sheets, and 'T' for traffic control plans.

The prefix 'X' does not correspond to any recognized category in the context of FDOT plan prefixes for sheet numbering. This makes it an outlier among the other options, which are actively used and recognized in the plan documentation process. Understanding these prefixes is essential for correctly interpreting and organizing the numerous sheets that comprise engineering plans, ensuring that professionals can easily reference and locate the necessary documents throughout a project's lifecycle.