Since 0.17.4, a pickup tower can work in either of two modes: unlimited mode and filtered mode.
Unlimited mode is same as the behaviour before (<= 0.17.3), a pickup tower try to pick every item on ground (within range), unless there is no space to contain the items.
From 0.17.4 onwards, a pickup tower WITHOUT circuit network signals (thus not connected by any circuit wires) will automatically work in unlimited mode.
Filtered mode allows limiting what type and amount of items can be picked up by a pickup tower. It is achieved by using circuit network signals as filters on each pickup tower, available since 0.17.4.
Here we have used loots from Schall Alien Loot as an example. There are lots of loots on the ground, where we want to set up the tower to automatically pick up the more valuable loot first. The alien artifact (round shaped, the fourth entry, in the below picture) is the most valuable. Among the three alien ores, the higher saturation in colour, the more valuable they are. The alien ore 1 (dark purple) is the least valuable.
The signals are used as filter, and work in whitelist mode.
To set up filter to a pickup tower, first connect a constant combinator to it by a circuit wire (red wire and/or green wire). (You may use complex circuit network to do so, but constant combinator is simplest so is used as demonstration here.)
Inside the constant combinator, click in the alien ores and alien artifact to include them as whitelisted items. Set the values to NEGATIVE of what you desired. It is like the "request setting" in a vanilla logistic requester chest, except a negative sign is needed here. Very important! A non-negative value is regarded as "fulfilled" request, so items of that type will not be collected. (Users of "LTN - Logistic Train Network" should find this concept familiar.)
In this example, we want the tower to pick up at most 1000 alien ore 1 and 3000 alien ore 2, to reserve more capacity for alien ore 3 and alien artifact.
When the pickup tower completes its scan, items are picked up to the storage as the above picture. All alien artifact and alien ore 3 are collected. Alien ore 1 and 2 are collected to exactly the set filter values. The extras are still on the ground, remain to be collected in another pickup cycle or another tower.
Do note that, however, the counts of items in storage ADD to the signal received by the tower. In the above picture, the signal of alien ore 1 is -1000 (from constant combinator) + 1000 (from stored items) = 0. It is non-negative so no alien ore 1 will be collected by this tower again, like a fulfilled logistic requester chest. So be sure to clear up the contents by inserters or loaders, like what you normally should do.