What does (06) in "CRL Signing (06)" mean? In "Certificate details -> key usage -> CRL Signing (06)", what does "(06)" in "CRL Signing (06)" mean?
The Key Usage field is represented as a bit string, where each bit represents one of the options, so that multiple options can be set. The value 6 it's just the constant of the bit position that identifies "CRL Signing". From RFC 5280:

KeyUsage ::= BIT STRING {
     digitalSignature        (0),
     nonRepudiation          (1),  -- recent editions of X.509 have
                                -- renamed this bit to contentCommitment
     keyEncipherment         (2),
     dataEncipherment        (3),
     keyAgreement            (4),
     keyCertSign             (5),
     cRLSign                 (6),
     encipherOnly            (7),
     decipherOnly            (8) }