You need to read the html documentation. Click on the classes tab. Select the SdFile class. Look at open().
Here is what you would find for open flags
[in] oflag Values for oflag are constructed by a bitwise-inclusive OR of flags from the following list
O_READ - Open for reading.
O_RDONLY - Same as O_READ.
O_WRITE - Open for writing.
O_WRONLY - Same as O_WRITE.
O_RDWR - Open for reading and writing.
O_APPEND - If set, the file offset shall be set to the end of the file prior to each write.
O_AT_END - Set the initial position at the end of the file.
O_CREAT - If the file exists, this flag has no effect except as noted under O_EXCL below. Otherwise, the file shall be created
O_EXCL - If O_CREAT and O_EXCL are set, open() shall fail if the file exists.
O_SYNC - Call sync() after each write. This flag should not be used with write(uint8_t), write_P(PGM_P), writeln_P(PGM_P), or the Arduino Print class. These functions do character at a time writes so sync() will be called after each byte.
O_TRUNC - If the file exists and is a regular file, and the file is successfully opened and is not read only, its length shall be truncated to 0.