Offering ID is typically used when installing a product. The listAvailablePackages command can be used to get the offering ID of a product. 

In this example, com.ibm.websphere.BASE.v90 is the offering ID. Following the offering ID is the version.release.modification.fixpack, such as 9.0.0.20160526_1854.

com.ibm.websphere.BASE.v90_9.0.0.20160526_1854
com.ibm.websphere.BASE.v90_9.0.1.20160906_0049
com.ibm.websphere.BASE.v90_9.0.2.20161108_1719
com.ibm.websphere.BASE.v90_9.0.3.20170217_1945
com.ibm.websphere.BASE.v90_9.0.4.20170523_1327
com.ibm.websphere.BASE.v90_9.0.5.20170918_1844
com.ibm.websphere.BASE.v90_9.0.6.20171205_1311

Offering ID is used in the XML file used by the imcl input command, like this.

<offering id='com.ibm.websphere.BASE.v90' 
  profile='IBM WebSphere Application Server V9.0' 
  features='core.feature,ejbdeploy,thinclient,embeddablecontainer' installFixes='none'/>