In a Spanish Social Security Historia de Vida Laboral under the heading Fecha, there are three subheadings: alta, efecto and baja. The dates for the first two are (in this statement) always the same. My assumption is that alta refers to the date in which the employee should have started according to her contract and efecto, to the date on which she actually started (unexpected illness and funerals, etc. notwithstanding). Apart from the space for the headings being restricted and not wordy in Spanish, I would opt for 'Contractual start date, actual start date and final day or left'. All comments and clarifications gratefully received.