Explanation:

Step 1 - Requirement
We need a custom sensitive information type (SIT) that detects:
An employee ID number in a specific format # hire date + three digits.
This is a pattern-based requirement.
Specific keywords within 300 characters of that ID # "Employee", "ID", or "Identification".
This is a keyword proximity requirement.
Step 2 - Sensitive info type elements in Microsoft 365
When creating custom SITs, you define primary and secondary elements:
Primary element # The main pattern or data to detect (the most defining factor).
Secondary element # Supporting evidence that must be found near the primary element to increase confidence.
Available elements:
Regular expression # Used to define patterns such as date + digits (for employee ID).
Keyword list # Used for lists of words/phrases like "Employee", "ID", "Identification".
Functions # Used for built-in validators like credit card checksum, not relevant here.
# Reference: Create a custom sensitive information type in Microsoft 365 Step 3 - Apply to scenario Employee ID pattern (hire date + 3 digits) # Needs a regular expression # This must be the Primary element.
Keywords ("Employee", "ID", "Identification") within 300 characters # A keyword list # This must be the Secondary element.