According to the IRS, the responsible party is an individual who directs and/or controls the entity behind the EIN, which means it is the one who signs the documents on behalf of the company and acts as the company's representative at the IRS. The responsible party must be an individual (that is, a natural person), not an entity.
All communication (hard copies and digital versions) will be mailed to the Responsible Party listed on the EIN form. A business's responsible party is, quite simply, the contact person within the organization for tax purposes.
Important to highlight that in an LLC, the responsible party MUST be one of the members for taxation purposes, which means that in a Single-Member LLC, the responsible party is the sole member.
You can find further information on the IRS website.
Can I change the responsible party later on?
Yes, you can change the responsible party for your company. However, two points need to be considered: