dissolve_option (Optional)
Specifies the dissolve to be performed to remove buffer overlap. NONE —An individual buffer for each feature is maintained, regardless of overlap. This is the default. ALL —All buffers are dissolved together into a single feature, removing any overlap. LIST —Any buffers sharing attribute values in the listed fields (carried over from the input features) are dissolved.
Buffer—Help | ArcGIS Desktop
The Dissolve Field(s) parameter Add Field button is used only in ModelBuilder. In ModelBuilder, where the preceding tool has not been run or its derived data does not exist, the Dissolve Field(s) parameter may not be populated with field names. The Add Field button allows expected fields to be added to the Dissolve Field(s) list in order to complete the Buffer tool dialog box.