In an earlier post on keyboards, I talked briefly about text service categories. I'd like to talk more about categories.
TSF will make sure that at most one text service in any category is enabled at any given time.
So, for example, you can enable one text service with GUID_TFCAT_TIP_KEYBOARD, one text service with GUID_TFCAT_TIP_SPEECH, and one text service with GUID_TFCAT_TIP_HANDWRITING, and all three text services will be active at the same time.
But if you enable another text service with GUID_TFCAT_TIP_KEYBOARD, the first one will be deactivated.
If you don't want this to happen, then you should call RegisterCategory with your own category GUID.