definition of pre-2011 and post 2011

The following information I copied from MS site https://docs.microsoft.com/en-us/microsoft-365/compliance/sensitive-information-type-entity-definitions?view=o365-worldwide#us-social-security-number-ssn.

Does it mean the SSN issued date is before 2011 or issued after 2011? To me it don't matter when the SSN is issued, it is just 9 digit number. the match accuracy should be the same. Did I miss something here?

Thanks Peter

U.S. social security number (SSN) Format

nine digits, which may be in a formatted or unformatted pattern

Pattern

four functions look for SSNs in four different patterns:

• Func_ssn finds SSNs with pre-2011 strong formatting that are formatted with dashes or spaces (ddd-dd-dddd OR ddd dd dddd)
• Func_unformatted_ssn finds SSNs with pre-2011 strong formatting that are unformatted as nine consecutive digits (ddddddddd)
• Func_randomized_formatted_ssn finds post-2011 SSNs that are formatted with dashes or spaces (ddd-dd-dddd OR ddd dd dddd)
• Func_randomized_unformatted_ssn finds post-2011 SSNs that are unformatted as nine consecutive digits (ddddddddd)