7.18.4.1 indique

The macro INTN_C(value) shall expand to an integer constant expression corresponding to the type int_leastN_t. The macro UINTN_C(value) shall expand to an integer constant expression corresponding to the type uint_leastN_t.

Sachant que (7.18.1.2)

The typedef name int_leastN_t designates a signed integer type with a width of at least N , such that no signed integer type with lesser size has at least the specified width. Thus, int_least32_t denotes a signed integer type with a width of at least 32 bits.
The typedef name uint_leastN_t designates an unsigned integer type with a width of at least N , such that no unsigned integer type with lesser size has at least the specified width. Thus, uint_least16_t denotes an unsigned integer type with a width of at least 16 bits.

la formulation (dans INT8_C):

[Description]
Appends the correct suffix to an 8-bit signed integer literal.

[Explanation] This macro appends the correct suffix for an 8-bit signed integer literal to <I>c</I>.

n'est pas précise, puisqu'un (u?)int_leastN_t pourrait faire plus de N bits. Il manque une mention de "on the TIGCC platform".