[ASN.1] doubts about context tag

[tong]

HI all

again GSM-MAP
example:
UpdateLocationArg ::= SEQUENCE {
                msi                     MSI,
                msc-Number      [1] ISDN-AddressString,
                vlr-Number      ISDN-AddressString,
                lmsi            [10] LMSI       OPTIONAL,
extensionContainer      ExtensionContainer      OPTIONAL,
        ... ,
        vlr-Capability  [6] VLR-Capability      OPTIONAL }

1 why encode 1,6,10,not continuously?
2 the order of context why not from small to big ? in this example is 1,10,6.
3 why some subtype(vlr-Number) not have context tag?
4 msc-Number not have OPTIONAL,so must presence,and so use context tag is absolutely 
unneeded ?