Please first read the "Mapping Parameter/Result Variables" section of the BPM Services documentation:
https://docs.jboss.org/author/display/SWITCHYARD/BPM+Services

Tooling clarifications:
1. Both the <parameters> and <results> sections are optional.
2. Both the <parameters> and <results> sections take 0..* <mapping> elements as children.
3. The "expression" and "variable" attributes of the <mapping> element are required for both <parameters> and <results>.
4. The "expression" value can be rooted via either a context, message or exchange.
5. The "expressionType" attribute of the <mapping> element is optional, however the only supported value at this time is MVEL.
6. The "contextScope" attribute of the <mapping> element is optional, however it can be IN, OUT or EXCHANGE. The default value is IN.