The ‘gain’ is the profit you make when you sell shares that have increased in value.
If your gain is above the annual exempt amount, you will need to report it to HMRC by either:
submitting a Self-Assessment tax return
using the ‘real time’ Capital Gains Tax service
If you use the ‘real time’ Capital Gains Tax service, you’ll need to submit a tax return as well if you need to report:
the sale of other shares in the same tax year
the sale of other chargeable assets
any reliefs you have claimed