Usage of the sign ':' in variable declaration

Question

from Microsoft docs : (https://learn.microsoft.com/en-us/windows/win32/debug/pe-format)

#define FRAME_FPO   0               
#define FRAME_TRAP  1
#define FRAME_TSS   2

typedef struct _FPO_DATA {
    DWORD       ulOffStart;            // offset 1st byte of function code
    DWORD       cbProcSize;            // # bytes in function
    DWORD       cdwLocals;             // # bytes in locals/4
    WORD        cdwParams;             // # bytes in params/4
    WORD        cbProlog : 8;          // # bytes in prolog
    WORD        cbRegs   : 3;          // # regs saved
    WORD        fHasSEH  : 1;          // TRUE if SEH in func
    WORD        fUseBP   : 1;          // TRUE if EBP has been allocated
    WORD        reserved : 1;          // reserved for future use
    WORD        cbFrame  : 2;          // frame type
} FPO_DATA;

what is the meaning of the sign ':' in the variable declarations ?

https://en.wikipedia.org/wiki/Bit_field#C_programming_language — Andreas Rejbrand, Feb 29 '20 at 20:30
This is a bit-field; there are some details available [in this post](https://stackoverflow.com/questions/35857111/c-how-to-use-bitfields). — nanofarad, Feb 29 '20 at 20:30

Usage of the sign ':' in variable declaration

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