I don't work, and had to rely on my husband's job(s) to sponsor my visas. He left his job of 15 years to start a new job about 6 weeks before my ILR application. Since we had no choice but to use his employment, and he'd not been with the new employer for at least six months, we had to go Category B, rather than Category A, for that application. The only difference is you would use 12+ months of payslips and 12+ months of bank statements. All other evidence is the same between the two categories. As Margo said, the employer letter comes from the employer at the time of application, to confirm your current and ongoing income.