The Ace of Cups is where Arthur Edward Waite's personal preoccupations are most visible, and knowing them changes how the card reads.
Waite was deeply invested in the Grail legends, wrote about them at length, and understood the mystical tradition he was reconstructing as fundamentally Christian. When Pamela Colman Smith drew this card in 1909 she produced a chalice with a dove descending towards it bearing a wafer marked with a cross, water falling into a lily pond, and the letter W inverted on the cup's face. That is a communion image and a Grail image, and it is unambiguous.
The older card is not. In the French pattern the Ace of Cups is an elaborate architectural vessel, more like a font or a reliquary than a chalice, and it carries no descending dove and no wafer. In the Mamluk deck from which the suit descends the sign is simply a cup. The Christian charge on this card enters in 1909, with a specific author, for a specific reason.
The suit's element in the Golden Dawn system is water, and the emotional and relational readings that dominate modern practice follow from that assignment rather than from anything older.
Read as it usually is, the card is an emotional beginning: love offered, a feeling that has not yet been spoken, sympathy, or an openness that has just become possible. Read through Waite's own imagery it is narrower and stranger, grace arriving unearned, from above, into a vessel whose only contribution is being open. Those two readings coexist peacefully in most practice, but they are not the same claim, and the second one is the one Waite actually intended.