Most law practitioners maintain a separate bank account for handling client monies. E.g. when conveyancing property, the buyer will deposit the necessary funds in the solicitor's client account (or trust account). Once all the formalities have been completed, the monies held in that account will be passed on to the vendor. It's essentially a simple form of escrow.

The use of 'stichting derdengelden' indicates that the firm in question uses a 'stichting' to manage client monies. I'm not familiar with the use of a derdengelden account to pay salaries, but no doubt the wider context will make things clear...

-> "SOLICITORS' ACCOUNTS REGULATIONS, 1955.
" Client " means any person on whose account a solicitor holds or receives client's money ;

" Client account " means a current or deposit account in a bank in the name of the solicitor in the title of which the word " client " appears ;

" Client's money " means money which a solicitor receives as a solicitor or agent or in connection with his practice as a solicitor on account of some other person, but does not include, (a) money to which he is, or in the case of a firm, one or more of the partners are, alone entitled ; or (b) money held or received on account of a trust of which the solicitor is a solicitor-trustee;"
http://www.irishstatutebook.ie/ZZSI218Y1955.html

-> "Trust bank accounts

3.1 A trust bank account is defined in the Rules as "any trust account at a bank in New Zealand either in the name of [a] solicitor or in the name of a firm in which that solicitor is a partner or is held out to be a partner".

3.2 Trust bank accounts must be designated "trust account", and you must put the bank on notice that the money in the trust bank account is trust money."
http://www.lawyers.org.nz/memsoltaccgl.asp#3a