Consulted with US customers (US-based) from abroad. Is this work within the US or did it take place outside the US?

[Vanessa A]

If you are abroad, meaning not physically present in the US, then the work is outside the US.  If you are a non-resident alien, then this income is generally not taxable to you.  However, if you are a US Citizen or Resident for tax purposes, then you would still need to include this income on your form 1040. Although you would still need to report the income, you may be able to claim a foreign tax credit or the foreign earned income exclusion. 

If you visited the US at all to do the work, then the work you performed while in the US would be inside the US. 

[rsharptrader]

Thanks for confirming; that was my interim best guess. 

-- American citizen living abroad doing a little consulting work for American friends back home