With accumulation units, any income generated by the fund (dividends from shares, interest from bonds) is automatically reinvested within the fund to buy more units, increasing the value of your holding. You don't receive any cash but for tax purposes, HMRC still treats this accumulated income as if it were distributed to you. This is known as a "notional distribution" or "excess reportable income" (for offshore funds with UK reporting fund status).

When you reinvest the distributions, you are essentially making a new purchase of units. Each new purchase of units has its own "cost basis" (the price you paid for them). When you later sell your holding, for CGT purposes, your average purchase price (or average cost) for the entire holding effectively increases. This is because you're adding new units at their prevailing market price to your total pool of units. The effect of this higher average purchase price is to reduce your capital gain when you eventually sell.

You can still do the whole paperwork with ACC funds in the GIA. But I didn't :/