Clickly on any of those links will take to you a call/meeting page with the status pre-set to accept, decline, etc. This is how the SugarCRM folks designed the system.
Ok, but the status of Call/Meetins are Planned, held, not held .... and not Accepct, Decline
I don't know the differences between the status of an Activity (Planned, held, not held) and the link that I can find in home apge: accept, decline, cancel....I've searched it in the SugarCRM documents but I haven't found it!
With the exception of only showing Planned activities on the home page, I don't think we attach any special meaning to any of the other status values.
The status values can mean anything you want. When you click Accept, you are taken to the edit page with the status set to Accept. The same is true for Decline, but as for what Decline means to you is up to you and your company.
I would imagine that the means are similar to those on Outlook. If someone sends you a meeting request and you Accept, it means that you will attend the meeting. If you Decline, then it means that you will not attend the meeting.