The Statement of Use (SOU) has to be submitted in those cases when an applicant applies for a trademark with an "intent to use (1b)" in the US.
In cases like this, the brand is not commercially active when the application is filed. In the US, it is required to be commercially active to finalize the trademark application. However, the applicant does not have to meet these requirements at the time of filing. It is sufficient to provide proof at a later date.
This is when the Statement of Use comes in. When a "Notice of Allowance" is issued (approximately 12 months from submitting the application), the applicant must provide proof that their goods/services are available in the US market.
The applicant will be formally requested to file a Statement of Use. The statement must include a specimen. It is proof of the commercial activity of a given brand in the US market. It can be, for example, a picture of the product or the packaging, an invoice, a screenshot of a website, marketing materials, etc. Before the Statement of Use is accepted, you will be required to pay a fee of $100 per class of goods and services.