When I was studying for CFA I remmember that they say when you get a margin call you have to bring up the margin to the intial level. I am working for a company now that allows clients to bring it up to the maintenence level only, and seems some only broker say they do the same.
What is the norm in the market?
Not sure about the client types and risk you manage, but at CME maintainance and initial may actualy be the same: http://www.cmegroup.com/clearing/risk-management/performance-bonds-margins.html
In addition, there may be aging of the margin calls. A helpful reference: http://www.nfa.futures.org/nfa-compliance/publication-library/margins-handbook.pdf
Edit - What I meant to say is: the level likely depends on the product, market condition and riskiness of the client. Just a couple of SP500 speculative long futures for a client holding cash at USD 100m should represent a different risk than for a client with just USD 100k cash.
Margins is basically a buffer money a client should have. Initial margin and maintenance margin itself is subjective when comes in practical world…broker will call for money for that much level so that money takes care of the exposure of broker on client …means allowed levels of margin of client. Risk management system some times define the level of margin for each script…I have seen brokers dealing in lowest margin also…this is possible for them because of thr risk management sys…hence you have this doubt.