ESOPs and RSUs create two separate, independent tax events in India, and the biggest mistake employees make is only planning for one of them. Exercise is the first: the moment you convert an option into an actual share, the spread between what you paid (the exercise price) and what the share was worth that day (the FMV) is treated as salary income, a "perquisite" under Section 17(2)(vi), and taxed at your normal slab rate. Your employer withholds TDS on it, exactly like a bonus.
This creates a well-known cash-flow trap for unlisted startup shares in particular: the tax bill is based on a paper valuation, but there's often no market to sell shares into to raise the cash to pay it. DPIIT-recognised startups holding a Section 80-IAC exemption certificate can offer their employees a deferral of that TDS, up to 48 months, or sooner if the shares are sold or the employee leaves, but it's a timing relief, not a tax reduction: the same amount comes due eventually.
Sale is the second event, and it's unrelated to the first: it's a capital gain calculated from the FMV on your exercise date (not your original exercise price) to your eventual sale price. Listed shares get the favourable Section 111A/112A treatment (20% flat short-term, or 12.5% long-term above a Rs 1.25 lakh exemption after 12 months). Unlisted shares need a longer 24-month hold to count as long-term, and even then get no exemption threshold, while short-term unlisted gains are taxed at your full slab rate rather than a flat rate.