Merge

The merge pattern allows you to combine data in the primary and secondary data paths into a single track. This pattern requires the source to be in the same class, conditions to match between primary and secondary sources, and how to handle data mismatches. The starting data point is the shape where you start merging data from. Once the merge path is available, you define the secondary source by providing an existing data set or another data flow.

Example: Merge shape combines the incoming transaction records with the customer records. The condition is that both records must have the same CustomerID property to be merged.

Compose

The compose pattern allows you to combine data from two sources into a page or page list property. This pattern requires a property to match data between the two sources. The starting data point is the shape where where you start combining data from. Once the compose path is available, you define the secondary source by providing an existing data set or another data flow.

Example: The merged records can be enriched with the information about other transactions

Refer to below article: https://community.pega.com/knowledgebase/articles/new-shapes-data-flow-form