why the "Planned Release" field in Requirement artifact must be of type integer?
Accepted answer
It's probably just using the integer data type, as defined in the project template you are using. You can configure what data type any attribute is using (so you could use an enumerated list for example). You can't change the data type of an existing attribute though, so you'd have to create a new attribute.
One other answer
If you need some help with creating artifact types and attributes here are some help topics:
Creating Requirement Attribute Data Types
https://jazz.net/help-dev/clm/index.jsp?re=1&topic=/com.ibm.rational.rrm.help.doc/topics/t_create_data_type.html&scope=null
Creating Requirement Attributes
https://jazz.net/help-dev/clm/index.jsp?re=1&topic=/com.ibm.rational.rrm.help.doc/topics/t_create_attribute.html&scope=null
Creating Requirement Types:
https://jazz.net/help-dev/clm/index.jsp?re=1&topic=/com.ibm.rational.rrm.help.doc/topics/t_create_artifact_type.html&scope=null
Creating Requirement Attribute Data Types
https://jazz.net/help-dev/clm/index.jsp?re=1&topic=/com.ibm.rational.rrm.help.doc/topics/t_create_data_type.html&scope=null
Creating Requirement Attributes
https://jazz.net/help-dev/clm/index.jsp?re=1&topic=/com.ibm.rational.rrm.help.doc/topics/t_create_attribute.html&scope=null
Creating Requirement Types:
https://jazz.net/help-dev/clm/index.jsp?re=1&topic=/com.ibm.rational.rrm.help.doc/topics/t_create_artifact_type.html&scope=null