OptimizeMember allows you to add custom fields to the registration forms when your users create their account on your site. This allows you to have additional fields in their user profile for information you wish to store for reference, for example company name, or some other important data you wish to collect.
To use this feature, go to OptimizeMember > General Options > Registration/Profile Fields & Options
From there you will see the option to "Add New Field" which will then trigger a popup window that allows you to create the settings for the additional field to collect data.
It's important to note that this field will show on the registration form when your user creates their account - it will not show on the checkout form which is designed for payment and not for user registration.