Short answer. Regular with respect to the activity you were actually employed in, and for as long as that activity exists. Article 280 ties the status to the work you did, not to the company at large, so it does not by itself give you a claim on some other job in the business.
What the law says
shall be considered a regular employee with respect to the activity in which he is employed and his employment shall continue while such activity exists
Labor Code, Article 280 — Regular And Casual Employment. Read the full provision →
The status is attached to an activity
The proviso in Article 280 is precise about what the year buys you. A casual who has rendered at least a year of service shall be considered a regular employee with respect to the activity in which he is employed and his employment shall continue while such activity exists. So the right you acquire is anchored to the work you were doing. If you spent the year as a warehouse checker, you are regular as a warehouse checker. The article is not conferring a general tenure in the enterprise divorced from any particular function.
Why the qualification is there at all
It is the price of a rule that applies to casual work. Casual employment, in the article's scheme, is work that is not usually necessary or desirable in the employer's main business — the very kind that can genuinely come to an end. Rather than pretend otherwise, the law gives the long-serving casual the protection of regular status inside the work actually being done. That is a real protection: it removes the annual pretence of a fresh hiring, and it means you can only be let go for a cause the Code recognises.
What 'while such activity exists' does and does not allow
This clause is sometimes read by employers as an escape hatch, as though renaming a section or reshuffling duties made the activity disappear. It does not. The activity ends when the work genuinely stops being done — not when the same tasks continue under a new job title, a new team, or a contractor brought in to do them. Where the work is still being performed for the company by somebody, the activity plainly exists, and your regular status with respect to it exists with it.
Proving what your activity was
Since everything turns on the work rather than the label, gather what describes it: your job order or assignment slip, the duties written on your identification card, schedules and rosters, supervisors' instructions, and any performance record. Where duties shifted over the year, note when and on whose instruction. And count your service carefully — the year may be continuous or broken, so short engagements separated by gaps still add together toward it.