Every object has various attributes that can be accessed via templates. For example, to access the address attribute of the client object, use {{ client.address }}
Client
name, address, address_2, city, state, zip, country, invoice_extra_fields, number, website, invoices, amount_outstanding, amount_unbilled, users
Invoice
invoice_items, subtotal, total_cost, total_tax, total_tax2, tax, tax2, po, total_payments, balance, transaction_balance, client, payments, currency_code,tax_label, tax2_label, number, due_date, date, title, note, paid?, paid_date, total_quantity, total_discount, total_gross, account, pure_balance, id, summary, cancelled?, total_pure_cost, total_late_fees, recipients, creator, sent?, billed_hours, extra_fields
Estimate
estimate_items, subtotal, total_cost, total_tax, total_tax2, tax, tax2, client, currency_code, tax_label, tax2_label, number, date, title, note, balance, scope, total_discount, total_gross, delivery_date, expiration_date, total_quantity, sent?, expired?, accepted?, revised?, resolved?, declined?, creator
Contract (Proposal)
date, author (author.name), author_signature, contract_signature (contract_signature.data, contract_signature.signer_name, contract_signature.created_at), subject, client, contract_sections
Contract Section
section_type ("freeform", "fees"), content (content.title, content.options)
Invoice Item
quantity, price, title, cost, projects, project_items, taxable?, secondary_tax?, created_at, section_header?, total_tax, section_subtotal, section_total
Estimate Item
quantity, price, title, cost, taxable?, created_at, section_header?, total_tax, section_subtotal, section_total
Project Item
title, description, hours, completion_date, user