IDA Help: op_offset

Convert operand to a complex offset expression
This is a more powerful version of op_plain_offset() function.
It allows to explicitly specify the reference type (off8, off16, etc)
and the expression target with a possible target delta.
The complex expressions are represented by IDA in the following form:
        target + tdelta - base

If the target is not present, then it will be calculated using
        target = operand_value - tdelta + base
The target must be present for LOW.. and HIGH.. reference types
     ea      - linear address of the instruction/data
     n       - number of operand to convert (the same as in op_plain_offset)
     reftype - one of REF_... constants
     target  - an explicitly specified expression target. if you don't
               want to specify it, use -1. Please note that LOW... and
               HIGH... reference type requre the target.
     base    - the offset base (a linear address)
     tdelta  - a displacement from the target which will be displayed
               in the expression.

success op_offset(long ea, int n, long reftype, long target, long base, long tdelta);

#define REF_OFF8    0              // 8bit full offset
#define REF_OFF16   1              // 16bit full offset
#define REF_OFF32   2              // 32bit full offset
#define REF_LOW8    3              // low 8bits of 16bit offset
#define REF_LOW16   4              // low 16bits of 32bit offset
#define REF_HIGH8   5              // high 8bits of 16bit offset
#define REF_HIGH16  6              // high 16bits of 32bit offset
#define V695_REF_VHIGH   7         // obsolete
#define V695_REF_VLOW    8         // obsolete
#define REF_OFF64   9              // 64bit full offset
                                   // note: processor modules or plugins may register additional
                                   // custom reference types (for example, REF_HIGHA16 is
                                   // used by MIPS, SPARC, PPC, ALPHA, TRICORE, etc.)
#define REFINFO_RVA         0x10   // based reference (rva)
#define REFINFO_PASTEND     0x20   // reference past an item
                                   // it may point to an nonexistitng address
                                   // do not destroy alignment dirs
#define REFINFO_NOBASE      0x80   // offset base is a number
                                   // implies that base have be any value
                                   // nb: base xrefs are created only if base
                                   // points to the middle of a segment
#define REFINFO_SUBTRACT  0x0100   // the reference value is subtracted from
                                   // the base value instead of (as usual)
                                   // being added to it
#define REFINFO_SIGNEDOP  0x0200   // the operand value is sign-extended (only
                                   // supported for REF_OFF8/16/32/64)
#define REFINFO_NO_ZEROS  0x0400  ///< an opval of 0 will be considered invalid
#define REFINFO_NO_ONES   0x0800  ///< an opval of ~0 will be considered invalid