Hi J W,
If the gains are from shares or securities from a company listed on a recognised stock exchange, shares in a UK open-ended investment company or units in an authorised unit trust; then they are entered in the listed shares and secturities section.
Where the gains are from Employee Shareholder Status, Enterprise Investment Scheme, Seed Enterprise Investment Scheme or Share loss relief where losses are claimed against income; those gains go in the 'unlisted shares and secturities' section.
Please have a look at the SA108 note here:
Capital Gains Tax summary notes
Thank you.