Hi
A 'cell' in this context would be something like {characteristic A, characteristic B, characteristic C, characteristic D} (including as many characteristics as the company chooses to use), where we could have, say:
A = product type
B = regular premium or single premium
C = initial, renewal, termination or investment expense
D = proportional to number of contracts, amount of benefit or amount of premium (etc)
Towards the bottom of page 18 of Chapter 30 the following example of a single cell is given: 'the initial expenses of product X which are proportionate to the sum assured'. So, in that particular case, the company is using the above characteristics A, C and D to form the cells (but not B).
In the example used in the Question on page 19 of that chapter, we see expense amounts being split into cells that are defined by just the characteristics A (2 choices) and C (3 choices), giving a total of 6 (=2x3) possible cells for expense allocation.
Hope that helps.