Custom fields let you capture information SPM doesn't track out of the box, such as a regulatory classification, a data classification, etc. You define a field for a record type, and records of that type can use it.
Create a field
Go to Configuration → Custom fields
Open the tab for the record type and select Add custom field.
Enter a Display name (required)
Choose the Value type:
Single-line text
Multi-line text
Number
Currency
Percentage
Date
Checkbox
Dropdown (single select)
Dropdown (multi-select) or
URL
Optionally fill in:
Description: explains the field to you and other admins
Sort order and Default value (optional)
Tooltip: shown to the person entering a value
Space: All Spaces (org default), or one Space so only records in that Space can use it
Category: groups related fields (Ungrouped by default)
Required
Field permissions: Who can see this field and Who can change the value, either Everyone with access or specific roles. As the dialog explains, these apply on top of access to the record itself: they can restrict a field further, never grant access to one, and admins always keep access.
Conditional Display: a rule for when the field appears.
Select Create.