This question is not datacentre specific (I don't think).

When a dedicated hosting company offers a server with a particular transfer limit per month, is this both ways or only downloading (outgoing) from the server?

I have always assumed it was only outgoing however a new application I have ben asked to wright will require VERY large files to be FTP'd to the server with only a small amount of web traffic.

Should I be including these daily FTP sessions in by data-transfer requirements or are companies less concerned about incoming bandwidth since for most people it's minimal?

It totally depends on the company, most will count bandwidth at the router if you have dedicated server etc...and therefore all data sent in or out is counted.

we measure our dedicated/co-lo customers as total bandwidth both in and out, by reading and logging the amount on that customers port on the switch.
but I have seen places that only read outgoing

Except in Aussie land where we only charge for incoming and outgoing is (dare I say it) unlimited hehehe. Well not really, but outgoing in un - metered.

We bill for the higher of either in or out, but not both. Our upstream, Inflow, in turn bills us in this way. So when we decide to offer DSL we will pay for incoming and get out free or visa versa with dedicated hosting. Hope this makes sense?

Drew

To clarify a bit, it's common for aggregate transfer billers to count all inbound and outbound traffic (in GB), whereas 95% metered billers typically only count the higher of either inbound or outbound.

Brandon