[ASN.1] When Sequence

[Upma_Gandhi]

Hi All,
    I am very new to this mailing list and also to ASN.1 syntax.
I just come across "when SEQUENCE"  syntax in one of the message type of
RASV4.
Following is a syntax of that message -

CapacityReportingSpecification ::= SEQUENCE
{
        when SEQUENCE
        {
                callStart       NULL OPTIONAL,
                callEnd         NULL OPTIONAL,
                ...
        },
        ...
}


 I was just wondering  about what is the significance of  "when" as a
prefix to SEQUENCE ?
Can anybody tell me about this. 

Thanks in advance
Upma