An undisclosed fee means the two clubs have agreed not to publish the price. Clubs do it to protect their negotiating position on future deals and to avoid setting a public benchmark. Figures reported in the press for undisclosed deals are estimates, not confirmed numbers.
Clubs have practical reasons. A published fee becomes the reference point for the next negotiation, for the player's own contract talks, and for supporters judging whether a signing has worked. Keeping it private removes all three.
The number still tends to emerge, usually from the clubs' published accounts a year or more later, and often at a different figure to the one reported at the time. Those accounts are the only version of a fee that anybody has signed off.
This is why fee reporting is one of the easiest places to get a story wrong. The ARCHV leads on the highest figure a named source has printed and does not round one up, invent one or repeat an aggregator's estimate as though it were a confirmed price.