I mean in my patch, the [?] first inlet in connected by the impulse osc (in fact the famous delta < 0 of a phasor) the second inlet is connected by a variable signal and the third by a feedback history, that's a kind of Sample and Hold no ?
The [?] operator is shorthand for [switch]. It's the C ternary operator. It tests for a condition (the first inlet). If the condition is true, it passes the second inlet's value. Otherwise, it passes the 3rd inlet's value.