Found it - from the "Employment After Retirement - How It Works For You" on the TRS website. It is a brochure. EAR is 'employment after retirement' and is explained in detail.

From the brochure:

Volunteering and Independent Contractor Services

If you perform work as an independent contractor or volunteer for a TRS-covered employer during the first 12 consecutive- calendar months after your effective retirement date and that work would normally be done by an employee, you will be considered an employee instead of a volunteer or independent contractor. This means your work will be subject to the EAR restrictions. Therefore, if you’re a service retiree who retired after Jan. 1, 2021 or a disability retiree, this could cause you to forfeit your annuity if your work does not qualify for an EAR exception. Furthermore, your work will restart your 12-month break in service required to be eligible for full-time employment without restrictions.

After the first 12 months following your retirement, you may volunteer or work as an independent contractor for a TRS- covered employer, without being subject to the EAR restrictions, if you meet all the legal requirements for volunteering or working as an independent contractor.

Working for a Third-Party Entity

A third-party entity is an entity retained by a TRS-covered employer to provide personnel to the institution to perform duties or provide services that an employee of the institution would normally perform or provide. If a TRS retiree is employed
by a third-party entity and the retiree performs services for or on behalf of a TRS-covered employer, the retiree will be considered employed in Texas public education for the purpose of EAR. This means the retiree’s work for the third-party entity will be subject to the EAR restrictions. Any employment with a third-party entity will also restart a retiree’s 12-month break in service required to be eligible for full-time employment without restrictions.

And call TRS.