When money is remitted out of India, banks are required to check whether appropriate tax has been accounted for on it before releasing the transfer. Forms 15CA and 15CB are the mechanism for that check: 15CA is a self-declaration the remitter files (often electronically), and 15CB is a chartered accountant's certificate confirming the tax position, required for larger or more complex remittances.
Which part applies
The specific requirement depends on the amount being remitted and whether the remittance is taxable in India at all. Small remittances, and remittances on a specified exempt list under Rule 37BB, can qualify for simplified treatment (Part A of Form 15CA, no 15CB needed). Larger taxable remittances typically need both the CA certificate (15CB) and the corresponding declaration (Part C of 15CA); a specified list of exempt categories can bypass the CA certificate requirement even at larger amounts (Part D).
Why this trips people up
The friction usually isn't the tax itself — it's the paperwork lead time. A CA certificate takes time to obtain, and banks won't process the transfer without it once it's required, so remittances planned around a deadline (a property closing, a tuition payment) can stall if the Form 15CB step is left until the last minute.
The Form 15CA/15CB checker on the NRE/NRO & TDS page estimates which part of the form applies to a given remittance amount and purpose, so you know what documentation to line up in advance.