LTE:What is the meaning of this marker " [[ ", "]]" in RRC Spec

Question

I have gone through the message in RRC Spec. In release 10 onwards they introduced a marker "[[ " , "]]" in the asn messages ...

What is the use importance of it and its meaning ?
see this example I have taken from spec...

Answer 1

Reference http://www.etsi.org/deliver/etsi_ts/136300_136399/136331/10.07.00_60/ts_136331v100700p.pdf
Page 102 - 103

Items in [[..]] represents the extention addition group and here is a explanation -

-- Example with extension addition group

ItemInfoList ::= SEQUENCE (SIZE (1..max)) OF ItemInfo 

ItemInfo ::= SEQUENCE { 
 itemIdentity INTEGER (1..max), 
 field1 Field1, 
 field2 Field2 OPTIONAL, -- Need ON 
 ... 
 [[ field3-r9 Field3-r9 OPTIONAL, -- Cond Cond1 
 field4-r9 Field4-r9 OPTIONAL -- Need ON 
 ]] 
} 

-- Example with traditional non-critical extension (empty sequence) 

BroadcastInfoBlock1 ::= SEQUENCE { 
 itemIdentity INTEGER (1..max), 
 field1 Field1, 
 field2 Field2 OPTIONAL, -- Need ON 
 nonCriticalExtension BroadcastInfoBlock1-v940-IEs OPTIONAL 
} 

BroadcastInfoBlock1-v940-IEs::= SEQUENCE { 
 field3-r9 Field3-r9 OPTIONAL, -- Cond Cond1 
 field4-r9 Field4-r9 OPTIONAL, -- Need ON 
 nonCriticalExtension SEQUENCE {} OPTIONAL -- Need OP 
} 

The UE shall, apply the following principles regarding the levels applicable in case of nested error handling:

• an extension additon group is not regarded as a level on its own. E.g. in the ASN.1 extract in the previous, a error regarding the conditionality of field3 would result in the entire itemInfo entry to be ignored (rather than just the extension addition group containing field3 and field4)

• a traditional nonCriticalExtension is not regarded as a level on its own. E.g. in the ASN.1 extract in the
  previous, a error regarding the conditionality of field3 would result in the entire BroadcastInfoBlock1 to be
  ignored (rather than just the non critical extension containing field3 and field4).