A very basic question about tuning-word of a DDS
Look at page 10 and further in the AD9850 DDS datasheet to see in what sequence the data needs to be presented to the DDS. Often a microcontroller is used to generate the signals as described in this datasheet.

It is similar to how you would write data to a memory chip. A certain sequence must be observed. This DDS is not very different in that respect.

I suggest to first familiarize yourself with the concept of communication over SPI and then look at the details of how that works on this DDS.

SPI just describes how the (data) bits get from A to B. The FQ, UD, CLKIN etc.. are just names for the registers in the DDS.

It also seems DDS has no memory needs that sequence nonstop as an input?

Fortunately the DDS does have memory, although they're called registers in essence these are just small memories. The DDS would be unable to function in a useful way without it. When you write settings, it uses those until you write new values or you switch the DDS off (obviously).\

There are some projects available on using an Arduino with the AD9850