Its usage has changed over the course of time, due to the ambiguity of the phrase. Most people don’t feel the need to respond, which makes it tough for the organizers to know whether some of their guests will attend the party or not. Regardless of the fact, it is important for the host to request invitees to RSVP for the event. This can take different forms, depending on your creativity. The practice will be carried forward by the host, who will need to prepare a guest list and choose a method for contacting each of them.
As a party invitation is considered a casual way of inviting guests, it is important that you keep that tone when asking someone whether they will be at the party or not. For instance, you could simply write kindly reply by the ( date), (Names of the people coming ), ----- accept or ---- regrets, before ending with something like, 'looking forward to celebrate with you'. In other cases you can leave blank spaces and ask the guest to respond on whether he or she will attend the party or not.
In special circumstances, one uses ‘RSVP, regrets only’ or simply ‘Regrets only’, which implies that only those will need to respond who will most certainly miss the party. This is the modern way of reducing communication between the organizers and the guest, with only those responding who are declining the invitation due to various reasons. All other guests will be expected to come to the party.