In any case, this lead to the following idea about mechanics:
SIN Qualities
The SIN Positive Quality
A character (presumably) has a natural SIN, which is based on their credit, purchase, and criminal history. This SIN is used whenever the character wishes to make a purchase. SINs are rated in the same manner as Lifestyles, as follows:
0 (0 BP) - SINless - the character has no legal SIN, or has erased all connections with their SIN for whatever reason.
1 (5 BP) - Criminal - the character has a "provisional" or criminal SIN, which isn't useful for much other than getting them in trouble. However, having a SIN at all is theoretically preferrable to being "outside the system". Using a Criminal SIN to make any purchases with an Availability rating above 4 will inevitably lead to trouble. The character has trouble borrowing money from anyone but loan sharks, but can get quick "payday loans" of up to 500¥ without jumping through too many hoops.
2 (10 BP) - Low - The character has a Poor credit rating. They can make purchases with this SIN of any gear with an Availability rating up to 8 (as long as they can afford it) without trouble, and can "legally" qualify for a Low lifestyle (up to 10 LP) without hassle. The character can extend a credit line of up to 2,000¥ at a time without hassle, or up to 200,000¥ at an Availability of 1 per 10,000¥ of total debt (including any unpaid prior credit).
3 (15 BP) - Medium - The character has an Average credit rating. They can make purchases with this SIN of any gear with an Availability rating up to 12 (as long as they can afford it) without trouble, and can "legally" qualify for a Medium lifestyle (up to 15 LP) without hassle. He can extend a credit line of up to 5,000¥ without difficulty, or up to 500,000¥ at an Availability of 1 per 25,000¥ of total debt (including any unpaid prior credit).
4 (20 BP) - High - The character has a Good credit rating. They can make purchases with this SIN of any gear with an Availability rating up to 16R (as long as they can afford it) without trouble, and can "legally" qualify for a High lifestyle (up to 20 LP) without hassle. The character can extend a credit line of up to 10,000¥ at a time without hassle, or up to 1,000,000¥ at an Availability of 1 per 50,000¥ of total debt (including any unpaid prior credit).
5 (25 BP) - High - The character has a Very Good credit rating. They can make purchases with this SIN of any gear with an Availability rating up to 20R (as long as they can afford it) without trouble, and can "legally" qualify for a High lifestyle (up to 25 LP) without hassle. The character can extend a credit line of up to 50,000¥ at a time without hassle, or up to 5,000,000¥ at an Availability of 1 per 250,000¥ of total debt (including any unpaid prior credit).
6 (30 BP) - Privileged - The character has a Perfect credit rating. They can make purchases with this SIN of any gear or lifestyle without trouble, even on credit. The character can extend a credit line of up to 100,000¥ at a time without hassle, or up to 10,000,000¥ at an Availability of 1 per 500,000¥ of total debt (including any unpaid prior credit).
Raising your Credit Score
Once a character has made a total number of purchases with a given SIN equal to that SIN's maximum credit rating without defaulting on a loan or flagging the attention of the authorities, they may invest Karma into raising their SIN's credit score to the next higher level (at which point the Zurich-Orbital tracking databases 'reset' their purchase history for the purposes of allowing credit extensions). If the character has the SINner Negative Quality at any level, they must buy it off before raising their SIN rating.
The SINner Negative Quality
A SINner is someone who is "in the system" and allows themselves to be tracked, collated, analyzed, and marketed to. In order to have the SINner Negative Quality, the character must have the SIN Quality. The SINner Negative Quality operates exactly like the Distinctive Style Flaw for the SIN in question. Whenever a character critically glitches an Availability roll, fails to pay a debt, or is captured and booked by a law enforcement agency, they gain the SINner Quality at a base level, or increase their existing rating of the Quality. (The GM may instead provide specific other qualities with an equivalent BP cost, such as Spammed, Wanted, Hung Out to Dry, In Debt, or Bad Rep) This Quality may be bought off during play, by scrubbing their data trail and hacking various databases. Once the character's SINner Negative Quality exceeds the Rating of their SIN, their SIN is effectively reduced by the difference until the credit or legal problems are dealt with. If a further critical glitch or arrest occurs, the character's SIN Rating may be permanently reduced by 1.
Fake SINs
Fake SINs also have a Credit Rating, in addition to their Veracity Rating. A fake SIN's Credit Rating cannot exceed the Veracity Rating. A Fake SIN costs (Veracity x Credit) x 1,000¥, with an Availability of (Veracity x Credit)F.