Final answer:
The placeholder for future code expansion in ICD-10-CM is represented by the letter 'X', which is used to fill in gaps within codes that require a seventh character extension but have fewer than 6 characters.
Step-by-step explanation:
In ICD-10-CM, a placeholder for future code expansion is shown as an X. The X is used as a placeholder at certain codes to allow for future expansion. An example of its application would be within a code that has fewer than 6 characters and a seventh character extension is required. In this case, the X must be used in the empty character spaces to fill in the gaps before the extension.