LFBLinkType's FromPortGroup definition?

In FE object LFB definition,when we describe LFB topology with LFBLinkTypes 
in which we define FromPortGroup as string the size of which is 
variable,should we use a fixed size string because when it is we have to 
use another FULLDATA TLV to encode each LFBLinkType? 
There are other places string type is used,I will not list them all.
             <name>LFBLinkType</name> 
               <synopsis> 
                 Link between two LFB instances of topology 
               </synopsis> 
               <struct> 
                 <element elementID="1"> 
                   <name>FromLFBID</name> 
                   <synopsis>LFB src</synopsis> 
                   <typeRef>LFBSelectorType</typeRef> 
                 </element> 
                 <element elementID="2"> 
                   <name>FromPortGroup</name> 
                   <synopsis>src port group</synopsis> 
                   <typeRef>string</typeRef> 
                 </element>

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