Two different kinds of placeholder live inside a document, and telling them apart is the single most useful thing to learn about the editor. Merge fields are the ones you fill in. Signer fields are the ones your recipient fills in.
What a merge field looks like:
Merge fields are lowercase words in curly braces, for example {provider_name}, {client_name}, {start_date}, {currency}, {amount} and {jurisdiction}. A sentence in a template might read: This agreement is between {provider_name} and {client_name}, dated {start_date}, for the sum of {currency} {amount}.
How you fill them:
The Merge Fields panel on the edit screen scans the document text and lists every merge field it finds, with a box for each. Type your values and click Apply Merge Fields. The values are written into the document text in place of the placeholders, so what you see in the editor after applying is exactly what your signer will read.
Why sending is blocked:
If any merge field is still present when you press Send for Signature, the send is refused and the message names the exact field that is missing, for example that the document still contains the unfilled merge field {amount}. A contract can never go out with a raw placeholder sitting in the middle of a payment clause.
Adding your own:
You are not limited to the ones a template ships with. Type any lowercase name in curly braces into the editor, using letters, digits and underscores, and it appears in the Merge Fields panel automatically the next time the panel refreshes.
Two fields worth using deliberately:
{jurisdiction} belongs in the governing law clause of every template, and it pays to be specific, naming the courts of a city and country rather than just a country. {currency} is deliberately separate from {amount} so that the same template can be used to bill in different currencies without editing the text.
Merge field values are sanitised and escaped when they are applied, so pasting a client name containing punctuation cannot break the document markup.