> ## Documentation Index > Fetch the complete documentation index at: https://docs.symmetry.com/llms.txt > Use this file to discover all available pages before exploring further. # Core concepts overview > The ideas behind Symmetry Payroll Forms that apply to every integration: what a form is, how SPF picks forms, how questions are served, and what you get back. These concepts hold no matter how you deliver Symmetry Payroll Forms (SPF), whether you call the API from your own interface, embed the Hosted UI, or run SPF On-Premise. Reading them once will make the quickstarts, configuration guides, and references easier to follow. ## How SPF works, end to end Every SPF integration moves through the same five stages. What differs is who performs each one: API clients do this work themselves, while Hosted and on-premise clients have SPF's own interface do it for them. The employee's home address and one or more work addresses are resolved to taxing jurisdictions, and SPF returns the federal, state, and local withholding forms that apply. In Choose Mode this step is skipped, because the employee selects a form directly. SPF serves each form as a series of question sets (one screen at a time) and tells you which screen comes next based on the answer the employee just gave. You send the employee's answers back to SPF, using the question IDs and option values exactly as SPF supplied them. SPF returns a completed, signed PDF of the withholding form along with a set of tax parameters: the employee's withholding elections in a structured, machine-readable form. You store the PDF according to your retention obligations and map the tax parameters into your payroll system so the employee is withheld correctly. Stage five is where SPF hands off. It establishes what the employee's elections *are*; turning them into a withholding amount is your payroll engine's job. See [What is SPF](/spf/overview/what-is-spf) for that boundary, and [SPF to STE tax parameter mapping](/spf/references/ste-tax-parameter-mapping) if you run both products. ## Forms and jurisdictions Before anything else, it helps to know what SPF means by "a form" and how it decides which ones an employee needs. Form IDs, names and titles, form types, and how locale variants relate to one another. How addresses resolve to jurisdictions, plus reciprocity, local forms, nexus, and lock-in letters. ## Collecting the employee's answers SPF drives the employee experience through two separate question systems: one that decides *which* forms are needed, and one that collects the answers for a given form. Choose, Guided, and Single Form mode, and how flow questions differ from form questions. Why SPF owns the question order, how required conditions work, and how a form reaches its end. ## What SPF gives you back The two outputs of a completed form, and how to read tax parameter IDs, values, and value types. How SPF versions forms, when a new version takes effect, and why prior versions are retained. ## Compliance and language The signature declaration every withholding form requires before it can be submitted. English, Spanish, and French support, and why Canadian French forms work differently. ## Where to go next Once these concepts are familiar, pick the quickstart that matches how you're delivering SPF. Build your own employee-facing interface on the SPF API. Embed the Symmetry-hosted forms UI in your application. Run the forms UI inside your own infrastructure.