You can prefill the signers of your documents with data coming from Salesforce. For that, you can use our SignRequest SetUp page on our Salesforce package.
If it's the first time you are creating your own Custom Object with prefilled_tags for a Salesforce Standard Object (Contacts, Lead or Opportunities, for example), you should open 'SignRequest SetUp' in Salesforce Lightning.
In the SignRequest SetUp page, you can find Advanced Settings, click on it.
You will find a box where you can prefill your document signers with data coming from your Salesforce Standard Objects, such as Contact, used in the example below.