New = Request Done
Progress=Approved;Not Approved
Pending=Pending for User;Pending for Brand
Closed=Cancelled;Completed

<apex:outputpanel layout="block">
    <apex:selectList label="Sub-Status" value="{!currentCase.SubStatus__c}" multiselect="false" size="1" required="false">
        <apex:selectOptions value="{!subStatusOptions}"/>
    </apex:selectList>
</apex:outputPanel>

public void caseSubStatusOptions(){
    if(currentCase.Category__c != null && currentCase.SubCategory__c != null){
        if(statusSubstatusMap.containsKey(currentCase.Status)){
            String subStatusStr = statusSubstatusMap.get(currentCase.Status);
            List<String> subStatusList = subStatusStr.split(';');
            subStatusOptions = new List<SelectOption>();

            for(String s : subStatusList){
                subStatusOptions.add(new SelectOption(s,s));
            }
        }
    }
}