A group is analogous to a table in a relational source or target definition. Most transformations have one input and one output group. However, some have multiple input groups, multiple output groups, or both. A group is the representation of a row of data entering or leaving a transformation.
All multi-group transformations are active transformations. You cannot connect multiple active transformations or an active and a passive transformation to the same downstream transformation or transformation input group.
Some multiple input group transformations require the Integration Service to block data at an input group while the Integration Service waits for a row from a different input group. A blocking transformation is a multiple input group transformation that blocks incoming data. The following transformations are blocking transformations:
Custom transformation with the Inputs May Block property enabled
Joiner transformation configured for unsorted input
When you save or validate a mapping, some mappings that contain active or blocking transformations might not be valid.