Credit Period vs. Compliance Period
The "credit period" is the 10-year period during which the owner claims its tax credits. It began either the same taxable year your building was placed in service or the following year.
The "compliance period" is the 15-year period during which you must follow the tax credit law to keep your site in compliance. It starts the same year the credit period does but lasts five years longer. This is because you must maintain compliance for 15 years, even though the owner claims its 15 years' worth of tax credits over just 10 years.