In the stdio.h (Standard IO facilities) of avr-libc there is a typedef for FILE. It is described as "the opaque structure that is passed around between the various standard IO functions". So perhaps FT_STATUS is used as an opaque structure that is passed around between various D2XX functions. In that case, ftStatus is a way to instantiate the structure into something more specific (hmmm... or less opaque). When I use the things that are made with FILE it is not really obvious that it is from the FILE typedef
stdout = stdin = uartstream0_init(BAUD);
but the function was defined as
FILE *uartstream0_init (uint32_t baudrate)
{...}
so some people like to use variable name notations as reminders. e.g. ft = FT_STATUS