报告描述符中的变量方式表示法要怎么去理解啊？

[Mcxpay]

int16_t index = (int16_t) (key / 8 + 1);
int16_t bitIndex = (int16_t) (key % 8);
hidBuffer[index + 1] |= 1 << (bitIndex);

a, b, c, d 理解起来都没问题，数值多少，就往前挪多少位。后面的为什么是在前面插入啊？看了半天描述，也没整明白这块。

Indicates whether the item creates variable or array data fields in reports. In variable fields,
each field represents data from a physical control.
The number of bits reserved for each field is determined by preceding Report Size/Report Count items.
For example, a bank of eight on/off switches could be reported in 1 byte declared by a variable Input item where each bit represents one switch,
on (1) or off (0) (Report Size = 1, Report Count = 8).
Alternatively, a variable Input item could add 1 report byte used to represent the state of four three- position buttons,
where the state of each button is represented by two bits (Report Size = 2, Report Count = 4).
Or 1 byte from a variable Input item could represent the x position of a joystick (Report Size = 8, Report Count = 1).

A: 0001 0000
B: 0010 0000
C: 0100 0000
D: 1000 0000
E: 0000 0000 0000 0001 => 0000 0001 0000 0000
F: 0000 0000 0000 0010
G: 0000 0000 0000 0100
H: 0000 0000 0000 1000
I: 0000 0000 0001 0000
J: 0000 0000 0010 0000
K: 0000 0000 0100 0000
L: 0000 0000 1000 0000
M: 0000 0000 0000 0000 0000 0001 => 0000 0001 0000 0000 0000 0000
M: 0000 0000 0000 0000 0000 0010