If the Balance of Payments (BP) is defined as $$BP=\text{Current Account}+\text{Capital Account}$$ where: $$\text{Current Account}=X-M+\text{Net Income Interest}$$ and $$\text{Capital Account}=\text{F}-\Delta \text{R}$$
Assume current account is positive, and $\text{Net Income Interest}=0$ for simplifying. Then there are 2 remaining ways for the balance of payments to be theoretically equal to zero:
1)$\text{F}<0$ and $\Delta R=0$ i.e. by home country acquiring foreign assets, or
2) $\text{F}=0$ and $\Delta R>0$ i.e. by the domestic Central Bank (CB) to increase their foreign exchange rate reserves.
Why would there be a need for official intervention by having the domestic central bank (CB) to increase its foreign reserves, if there was no buying of foreign assets ($\text{F}=0$), of ?
Any help would be appreciated.