Two ways to put money into the same fund
A SIP invests a fixed amount at regular intervals - usually monthly. A lumpsum invests everything at once, on a single date. Both can go into the exact same fund; the difference is entirely about timing and discipline, not about the fund itself.
What a SIP is actually doing for you
Investing a fixed amount every month means you buy more units when prices are lower and fewer units when prices are higher - a pattern often called Rupee Cost Averaging. Over time, this averages out your purchase cost, and it removes the need to guess whether “now” is a good time to invest, which is one of the hardest calls to get right consistently.
SIPs also build a habit. For most salaried investors, that habit - not any clever timing - is what actually determines whether a goal gets funded.
When a lumpsum genuinely fits better
If you’ve received a bonus, matured an FD, or come into a windfall, and you have a long time horizon before you need the money, a lumpsum can put that capital to work immediately instead of trickling it in over months. The trade-off is that a lumpsum is fully exposed to whatever the market is doing on the day you invest - there’s no averaging cushion.
A common middle path for a large sum: split it into a Systematic Transfer Plan (STP) that moves it from a liquid fund into an equity fund gradually, combining some of the averaging benefit of a SIP with capital that’s already set aside and working.
The honest answer
For ongoing income - your salary - SIP is the natural fit, because that’s how the money arrives in the first place. For a one-time sum you already have in hand, the right choice depends on your time horizon and comfort with short-term swings, not a universal rule.