true when the value has none of the type-specific prefixes (internal) Period and phase are encoded:

• The period of validity from the block hash found in the signing material.
• The phase in the period that this transaction's lifetime begins (and, importantly, implies which block hash is included in the signature material). If the period is greater than 1 << 12, then it will be a factor of the times greater than 1<<12 that period is.