I don't know how it works in other councils but in mine an earmarked reserve is simply a reserve that has been built up for a specific purpose in mind. For example we built up an earmarked reserve to replace play equipment in the recreation ground.
Expenditure from reserves is treated no differently from any other expenditure in that it is allocated to a budget heading as an accounting activity, and where expenditure is against an earmarked reserve, the expenditure is allocated to that budget heading in the same way as any other expenditure.
Where committees have budgetary authority to agree expenditure then the same rules apply wherever the money comes from (e.g. earmarked reserve, precept or general reserve) so same process for tendering where applicable, etc. etc. In the example of our play equipment, our parks committee have delegated authority to agree expenditure up to (in our case) £5K per contract/item. Our EMR was £10K and after going through the tender process, the quotes came in at around the £9K mark, i.e. over the limit the committee could agree. This was then referred to full council as a recommendation to accept a quotation based upon the tendering process. Full Council authorised the expenditure and equipment was ordered and installed. When the invoice came in, it again went to full council (as is our procedure), approved for payment and paid in the usual manner. The RFO then allocated the expenditure against the EMR in the accounts leaving a balance at the year end which, on agreement, was retained as an EMR to fund future developments of the play park.