SalesInbox Options when Sending Email

SalesInbox Options when Sending Email

Is there any documentation that explains why certain options are sometimes available when sending email and why those options are sometimes not available? For example...

1. Send at a later date/time (also would be nice if users could change the date/time)
2. Add Response Watch
3. Add Task
4. Add Note

The above options are sometimes shown but sometimes not and I would like to know the reason behind that. Thank you.