• Start create or edit an action.
• Select $Step context object (1).
• Select item from context object items picklist (2).

$Step context object stores values of a current process step. On process actions with "Before Step" type step items store values of a previous process step, otherwise Step items refer to a current step.

• ApiName - current step API Name.
• AssignTo - current step assignee. Value depends on Step.AssignType: if step assign type is User, Group, ChatterGroup, Queue, Role, Profile or Resource assignTo will store corresponding record IDs, if assign type is "Call Apex" assignTo will store Apex Class Name, if assign type is "Field" assignTo will store field API Name.
• AssignType - type of a current step assignment. Can store one of the following values: User, Field, Group, ChatterGroup, Queue, Role, Profile, Resource, Call Apex.
• Branch - label of a corresponding process branch.
• Branch.Id - ID of a corresponding process branch.
• Description - current step description.
• Duration - duration of current step.
• Effort - current step effort.
• Guid - current step GUID.
• Id - current step ID.
• Milestone - step milestone name.
• Milestone.Id - step milestone ID.
• Name - step name.
• Swimlane  - step responsibility name.
• Swimlane.AssignTo - responsibility assignee value.
• Swimlane.AssignType - responsibility assignment type. Can store one of the following values: User, Field, Group, ChatterGroup, Queue, Role, Profile, Call Apex.
• Swimlane.Id - current step responsibility ID value.
• Type - type of a current step. Will store one of the following values: Manual, Approval, Automatic.