Record types and picklist values
Key Takeaway
Adding a value to a field's master picklist doesn't add it to existing record types. Users can't see the new value until you add it to their record type.
Record types let one object offer different picklist values, page layouts, and business processes to different groups of users. Every record type takes its picklist values from the field's master picklist, but each record type keeps its own list of the values it shows. Most scenarios about a picklist value that users can't see come down to how these two lists relate.
What record types can separate
For example, you can separate regular sales deals from professional services engagements by giving them different picklist values. You can also show different page layouts for customer support cases and billing cases.