There are two modes for product mapping and mapped history modes: End-item only and All Items. You can select your preferred Product Mapping mode on the The Advanced Tab of the Options dialog.
In End-items only mode, mapped history can only be assigned at an end item level. The mapped history is then used to create a forecast history at the end-item level. End-item forecast histories are aggregated to group levels. That is, the group level forecast history will be defined as the sum of forecast histories across all items that are included in the group. If you are working in End-items only mode, the Product Mapping icon will not be available on the Forecasting tab when a group level item is selected on the Navigator.
In All levels mode, you may assign mapped history to both end-items and groups, but forecast histories will not be aggregated to higher level groups or disaggregated to lower levels of the hierarchy. All groups and items must be explicitly assigned mapped histories.
If you plan on doing all product mappings at the lowest level, you should work in End-item only mode. Doing so will make sure you automatically aggregate forecast histories to the group level and generate group-level forecast histories.
If you need to create product mappings at a higher level in your hierarchy, you will need to work in All levels mode. You should only create a product mapping for a group item if you plan on using a reconciliation modifier (\TOPDOWN, \CALL or \INDEXES) for that group. If you do not plan on using a reconciliation modifier, the group level forecast history is not used to generate your forecasts, so assigning mapped history is not necessary. Please see Group Level (Reconciliation Modifiers) for more details.