A subpoena compels a non-party to produce documents or give testimony. The rules impose an affirmative duty to avoid imposing undue burden, and courts enforce it more readily than between parties.
Notice to the other side before service is required in most systems, so that they may object. Skipping it is a procedural error that can invalidate the production.
Geographic limits on where a non-party can be compelled to appear are strict. Document-only subpoenas are usually easier, and remote deposition arrangements can solve much of the rest.
Cost. A non-party may seek reimbursement of significant expenses of compliance, and offering to pay reasonable costs at the outset converts an adversarial exchange into a cooperative one. It is usually cheaper than the motion practice avoided.
Objections and motions to quash. A recipient may serve written objections within the stated period, shifting the burden to the issuing party to move to compel. Grounds include privilege, undue burden, insufficient time, and trade secrets.
Strategic use. Bank records, accountants’ files, counterparties’ communications and industry comparators often prove points that the opposing party’s own production will never concede. Non-party discovery is under-utilised, and it is frequently where a commercial case is actually made.