TC1 - Global, encodes NThe ISO 7816-3 specification is not public. So I can't copy/paste part of the text. I will use Wikipedia instead.
From Wikipedia https://en.wikipedia.org/wiki/Answer_to_reset#Interface_byte_TC1 (with some edition to remove extra details):
TC1, if present, is global, and encodes the Extra Guard Time integer (N), from 0 to 255 (8th MSbit to 1st LSbit); otherwise, N = 0. N defines how much the Guard Time that the reader must apply varies from a baseline of 12 ETU (corresponding to 1 start bit, 8 data bits, 1 parity bit, and 2 stop bits; with the second stop bit possibly used for an error indication by the receiver under protocol T = 0). The Guard Time is the minimum delay between the leading edge of the previous character, and the leading edge of the next character sent.
Except when N is 255, the Guard Time is: GT = 12 ETU + R*N/f
N = 255 has a protocol-dependent meaning: GT = 12 ETU during PPS (Protocol and Parameters Selection) and protocol T = 0, GT = 11 ETU under protocol T = 1 (corresponding to 1 start bit, 8 data bits, 1 parity bit, and 1 stop bit; with no error indication).
- f is the clock frequency being generated by the reader;
- R is some number of clock cycles, either:
- per ETU, R = F/D, if T = 15 is absent from the ATR;
- defined by TA1, R = Fi/Di (or its default value), if T = 15 is present in the ATR.
Except under protocol T = 1, the card transmits with a Guard Time of 12 ETU, irrespective of N. Under protocol T = 1, the Guard Time defined by N is also the Character Guard Time (CGT), and applies to card and reader for characters sent in the same direction.
Note: The reader remains bound by the Guard Time GT defined by N when other prescriptions specify another minimum delay between leading edges of characters in different directions, even when that minimum is lower than GT.
Historical note: ISO/IEC 7816-3:1989 only defined that N code the EGT as a number of ETU, the method now used when T = 15 is absent from the ATR. With this convention, cards that allow negotiation of a reduced number of clock cycles per ETU after PPS must also allow a proportionally reduced number of clock cycles for the EGT, which does not match with a common EGT motivation: account for delays before the card can receive the next character. The 1997 edition of the standard introduced that when T = 15 is present in the ATR, N code the EGT as a multiple of the number of clock cycles per ETU coded by TA1, making the EGT effectively independent of the number of clocks cycles per ETU negotiated, while maintaining compatibility with former readers at least if they did not change the number of clock cycles per ETU.
To make it short TC1 defines a time value between characters transmitted between the card and the reader. The default value correspond to a Guard Time of 12 ETU. But the card can specify a bigger value (the card is not fast enough).
The default value is 0. But 38.22% of cards explicitly define TC1=0.
A total of 82.14% of cards use the default value.
TC1=255 is also a special value not far from the default value of 12 ETU.
The real number of cards that are not fast enough to use the full speed is only of 4.00% (84 cards).